Voltage testing and measurement

ABSTRACT

A method of testing a device under test comprising providing a beam of light from a light source having a first wavelength. Imposing the beam of light on a test device over a spatial region within the test device substantially greater than the first wavelength, wherein the test device has a first state of refraction. Imposing the beam of light on the test device over a spatial region within the test device substantially greater than the first wavelength, wherein the test device has a second state of refraction. Obtaining data resulting from the interference of the first beam and the second beam within the device under test representative of the voltages within the region, wherein the first state of refraction is at a first voltage potential, and wherein the second state of refraction is at a second voltage potential different from the first voltage potential.

This application is a continuation of and claims priority of U.S. patentapplication No. 09/626,420, filed on Jul. 26, 2000, now U.S. Pat. No.6,512,385, issued Jan. 28, 2003; which claims the benefit of U.S.Provisional Patent Application Ser. No. 60/145,617, filed Jul. 26, 1999.

BACKGROUND OF THE INVENTION

The development of advanced integrated circuit devices and architectureshas been spurred by the ever increasing need for speed. For example,microwave, fiber optical digital data transmission, high-speed dataacquisition, and the constant push for faster digital logic in highspeed computers and signal processors has created new demands onhigh-speed electronic instrumentation for testing purposes.

Conventional test instruments primarily include two features, theintegrated circuit probe that connects the test instrument to thecircuit and the test instrument itself. The integrated circuit probe hasits own intrinsic bandwidth that may impose limits on the bandwidthachievable. In addition, the probe also determines an instrument'sability to probe the integrated circuit due to its size (limiting itsspatial resolution) and influence on circuit performance (loading of thecircuit from its characteristic and parasitic impedances). The testinstrument sets the available bandwidth given perfect integrated circuitprobes or packaged circuits, and defines the type of electric test, suchas measuring time or frequency response.

Connection to a test instrument begins with the external connectors,such as the 50 ohm coaxial Kelvin cable connectors (or APC-2.4). Theintegrated circuit probes provide the transitions from the coaxial cableto some type of contact point with a size comparable to an integratedcircuit bond pad. Low-frequency signals are often connected with needleprobes. At frequencies greater than several hundred megahertz theseprobes having increasing parasitic impedances, principally due to shuntcapacitance from fringing fields and series inductance from long, thinneedles. The parasitic impedances and the relatively large probe sizecompared to integrated circuit interconnects limit their effective useto low-frequency external input or output circuit responses at the bondpads.

Therefore, electrical probes suffer from a measurement dilemma. Goodhigh-frequency probes use transmission lines to control the lineimpedance from the coaxial transition to the integrated circuit bond padto reduce parasitic impedances. The low characteristic impedance of suchlines limits their use to input/output connections. High-impedanceprobes suitable for probing intermediate circuit nodes have significantparasitic impedances at microwave frequencies, severely perturbing thecircuit operation and affecting the measurement accuracy. In both cases,the probe size is large compared to integrated circuit interconnectsize, limiting their use to test points the size of bond pads. Likewisesampling oscilloscopes, spectrum analyzers, and network analyzers relyon connectors and integrated circuit probes, limiting their ability toprobe an integrated circuit to its external response. For networkanalysis, a further issue is de-embedding the device parameters from theconnector and circuit fixture response, a task which grows progressivelymore difficult at increasing frequencies.

With the objective of either increased bandwidth or internal integratedcircuit testing with high spatial resolution (or both) differenttechniques have been introduced. Scanning electron microscopes or E-beamprobing uses an electron beams to stimulate secondary electron emissionfrom surface metallization. The detected signal is small for integratedcircuit voltage levels. The system's time resolution is set by gatingthe E-beam from the thermionic cathodes of standard SEM'S. Fordecreasing the electron beam duration required for increased timeresolution, the average beam current decreases, degrading measurementsensitivity and limiting practical systems to a time resolution ofseveral hundred picoseconds. Also, SEM testing is complex and relativelyexpensive.

Valdmanis et al., in a paper entitled “Picosecond Electronics andOptoelectronics”, New York: Springer-Verlag, 1987, shows anelectro-optic sampling technique which uses an electrooptic lightmodulator to intensity modulate a probe beam in proportion to a circuitvoltage. Referring to FIG. 1, an integrated circuit 10 includes bondedelectrical conductors 12 fabricated thereon whereby imposingdifferential voltages thereon gives rise to an electric field 14. Forcarrying out a measurement an electro-opti needle probe 16 includes anelectro-optic tip 18 (LiTaO₃) and a fused silica support 20. A lightbeam incident along path 22 is reflected at the end of the electro-optictip 18 and then passes back along path 24. An electric field 14 altersthe refractive index of the electro-optic tip 18 and thereby alters thepolarization of the reflected light beam on the exit path 24, which thusprovides a measure of the voltages on the conductors 12. Unfortunately,because of the proximity of the probe 16 to the substrate 10 capacitiveloading is applied to the circuit, thereby altering measurementstherefrom. In addition, it is difficult to position the probe 16 inrelation to the conductor because the probe 16 and circuit 10 arevibration sensitive. Also, the measurements are limited to conductors 12on or near the surface of the circuit 10. Further, the circuit must beactive to obtain meaningful results and the system infers what isoccurring in other portions of the circuit by a local measurement

Weingarten et al. in a paper entitled, “Picosecond Optical Sampling ofGaAs integrated Circuits”, IEEE Journal of Quantum Electronics, Vol. 24,No. 2, February 1988, disclosed an electro-optic sampling technique thatmeasures voltages arising from within the substrate. Referring to FIG.2, the system 30 includes a mode-locked Nd:YAG laser 32 that providespicosecond-range light pulses after passage through a pulse compressor34. The compressed pulses are passed through a polarizing beam splitter36, and first and second wave plates 38 and 40 to establishpolarization. The polarized light is then directed at normal incidenceonto an integrated circuit substrate 42. The pulsed compressed beam canbe focused either onto the probed conductor itself (backside probing) oronto the ground plane beneath and adjacent to the probed conductor(front-side probing). The reflected light from the substrate is divertedby the polarizing beam splitter 36 and detected by a slow photo diodedetector 44. The photo diode detector is also connected to a display 46.

A microwave generator 48 drives the substrate 42 and is also connectedto an RF synthesizer 50, which in turn is connected to a timingstabilizer 52. The pulse output of the laser 32 is likewise connected tothe timing stabilizer 52. The output of the stabilizer 52 connects backto the laser 32 so that the frequency of the microwave generator 46locks onto a frequency that is a multiple of the laser repetition rateplus an offset. As a consequence, one may analyze the electric fieldsproduced within the integrated circuit as a result of being voltagedrive, thus providing circuit analysis of the integrated circuitoperation. In essence, the voltage of the substrate imposed by themicrowave generator 48 will change the polarization in the return signalwhich results in a detectable change at the diode detector 44.

Referring to FIGS. 3A and 3B, the locations along the incident beam aredesignated a, b, c (relative to the “down” arrow), and designated alongthe reflected beam as d, e, and f (relative to the “up” arrow), and theintensity modulated output signal is designated as g. The correspondingstates of polarization exhibited in the measurement process are shown inthe similarly lettered graphs of FIG. 3B. At location a of FIG. 3A, thepolarizing beam splitter 36 provides a linearly polarized probe beam (asshown in graph a of FIG. 3B) that is passed through the first wave plate38, which is a T/2 plate oriented at 22.5 degrees relative to theincident beam polarization, so as to yield at location b the 22.5 degreeelliptically polarized beam shown in graph b of FIG. 3B). The beam thenpasses through the second wave plate 40, which is a T/2 plate orientedat 33.75 degrees relative to the incident beam, so as to rotate the beaman additional 22.5 degrees to yield at location c the 45 degreepolarization (shown in graph c of FIG. 3B), which is at 45 degrees tothe [011] direction of the substrate 42, i.e., the cleave plane of thewafer. Similar rotations are shown for the reflected beam at thesuccessive locations d, e, and f, the resultant polarizationsrespectively being as shown in graphs d, e, and f of FIG. 3B As shown ingraph f in particular, the electro-optic effect of any voltage presenton the substrate 42 at the spot at which the beam reflects therefrombrings about a change in the specific polarization orientation in anamount designated in graph f of FIG. 3B as &, and that change isreflected in an amplitude change or intensity modulation in the outputsignal at location g that passes to the photo-diode 44 (as shown ingraph g of FIG. 3B). It is the measurement of & that constitutes thevoltage measurement. Among the various techniques of pre-determining thevoltage patterns to be used in testing an integrated circuit, or indeedan entire printed circuit, Springer, U.S. Pat. No. 4,625,313, describesthe use in a CPU of a ROM “kernel” in which are stored both a testprogram sequence and the testing data itself.

Since the system taught by Weingarten et al. does not include a probeproximate the circuit under test the limitations imposed by capacitiveloading of the circuit to be tested is avoided. However, the systemtaught by Weingarten et al. is limited to “point probing,” by the lens41 converging the input beam into a test point on the order of onewavelength. Unfortunately, to test an entire circuit an excessive numberof tests must be performed. In addition, it is not possible to testmultiple points simultaneously without the use of multiple systems,which may be useful in testing different portions of the circuit thatare dependant upon one another. The resulting data from the system ispresented to the user as a single amplitude measurement, i.e., theintensity of the signal produced at the photo-diode 44 depends simplyupon the degree to which the polarization of the reflected lightentering the beam splitter 36 has been rotated, so that not only are theactual phase and polarization data that derive the reflection processlost, but the precision and accuracy of the measurement becomes subjectto the linearity and other properties of the photo-diode 44 and thedisplay 46.

Various other techniques by which semiconductors may be characterized,using electromagnetic radiation of different wavelengths under differentconditions is cataloged by Palik et al. in “Nondestructive Evaluation ofSemiconductor Materials and Device,” Plenum Press, New York, 1979,chapter 7, pp. 328-390. Specifically, treatment is given of (1) infraredreflection of GaAs to obtain the optical parameters n and k and then thecarrier density N and mobility u; (2) infrared transmission in GaAs todetermine k from which is determined the wavelength dependence of freecarrier absorption; (3) infrared reflection laser (spot size) scanningof and transmission through GaAs to determine free carrier density inhomogeneity, including local mode vibrations; (4) far infrared impurityspectra; (5) infrared reflection and transmission from thin films on aGaAs substrate; microwave magnetoplasma reflection and transmission; (6)submillimeter-wave cyclotron resonance in GaAs to determinemagnetotransmission; (7) ruby laser radiation to form a waveguide in aGaAs film on a GaAs substrate, the propagation features of which arethen measured using infrared radiation; (8) infrared reflectance frommultilayers of GaAs on a GaAs substrate; (9) reflectance measurements ofgraded free carrier plasmas in both PbSnTe films on PbSnTe substratesand InAs on GaAs substrates; (10) interferometric measurements of ionimplanted layers; (11) infrared restrahlen spectra, also to determinelattice damage effects; (13) ellipsometric measurements of ion-implantedGaP; (14) determination of optical constants by internal reflectionspectroscopy; (15) laser raster scanning of semiconductor devices tomeasure photoconductivity, to track the flow of logic in a MOS shiftregister (because of current saturation, the effect of the laser lightdiffers in cells in the 0 or 1 logic state), and with a more intenselaser power level to change those logic states (i.e., to write to thecircuit); (16) laser raster scanning of semiconductor devices todetermine variations in resistivity and carrier lifetimes; (17) thermalimaging of circuits to find hot spots; (18) Raman backscattering todetermine free carrier density; (19) carrier injection to study the bandedge; (20) birefringence measurements in monolayers of GaAs and AlAs onGaAs to characterize the resultant strain; (21) photoluminescence andcathodoluminescence measurements of implanted layers and acceptor anddonor densities. With the exception of (7) above which relates towaveguide transmission, and also of (15) and (17), these techniquesrelate to the characterization of static systems. While (15) relates toa spot scanning technique of the operational integrated circuit and (17)relates to hot-characterization of the device temperature.

What is desired, therefore, is a non-invasive technique to measurevoltage levels within a device.

BRIEF DESCRIPTION

FIG. 1 is illustrates an electro-optic sampling technique.

FIG. 2 illustrates a voltage measurement system.

FIGS. 3A and 3B illustrate polarization changes.

FIG. 4 illustrates a holographic system.

FIGS. 5A and 5B illustrate holographic systems.

FIGS. 6A-6E illustrate crystal structures.

FIG. 7 illustrates a holographic apparatus.

FIG. 8 illustrates a holographic apparatus.

FIG. 9 illustrates transmission and reflection structures.

FIG. 10 illustrates an off-axis technique.

FIG. 11 illustrates an off-axis technique.

FIG. 12 illustrates a transmissive based system.

FIG. 13 illustrates an interferometric tester.

FIGS. 14A-C illustrates testers.

FIGS. 15A-15B illustrate testers

FIG. 16 illustrates thermoplastic recording.

FIG. 16-16C illustrate interferograms.

FIG. 17 illustrate interference fringes.

FIG. 18 illustrate interference fringes.

FIG. 19 illustrate interference fringes.

FIG. 20 illustrate a recording structure.

FIG. 21 illustrate a recording structure.

FIG. 22 illustrate a recording structure.

FIG. 23 illustrate a recording structure.

FIG. 24 illustrate a recording structure.

FIG. 25 illustrate a recording structure.

FIG. 26A illustrate a recording structure.

FIG. 26B illustrate a recording structure.

FIG. 27A illustrate a recording structure.

FIG. 27B illustrate a recording structure.

FIG. 28 illustrate a testing apparatus.

FIG. 29 illustrates a photo-conductor.

FIG. 30 illustrates a barrier.

FIG. 31 illustrates a photo-conducting crystal.

FIG. 32 left blank.

FIG. 33 illustrates photo-currents.

FIG. 34 left blank.

FIG. 35 illustrates a testing apparatus.

FIG. 36 illustrates a testing apparatus.

FIGS. 37A-37B illustrate system components.

FIG. 38 illustrate a testing apparatus.

FIG. 39 illustrate interference patterns.

FIGS. 40A-40B illustrate interference patterns.

FIG. 40C illustrates a setup.

FIG. 41A-41B illustrates test output.

FIG. 42 illustrate interference output.

FIG. 43 illustrate diffraction.

FIG. 44 illustrate a test setup.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present inventor came to the realization that the single pointnon-invasive probing technique of semiconductor materials could beenhanced if both the phase and amplitude, or polarization and amplitudeproperties of light transmitted thorough or reflected off of asemiconductor material could be recorded or otherwise preserved in somemanner. Semiconductor materials generally exhibit electro-optic(generally 3/5 semiconductor materials) or photo-refractive effects(generally silicon based expitaxal circuits), which can be made tobecome birefringent by the application of an electric field, either assuch or as embodied in electromagnetic radiation. Also, surfacereflection and/or transmission probing, of semiconductor materials, suchas for example, GaAs, germanium or silicon, can be modulated by forexample, cathode reflections modulation (E-beam), an electric field,voltage, heat, pressure, x-ray radiation, magnetic fields, and photoinjection. The present inventor then came to the realization that if anobject in a state in which it is not birefringent, but suchbirefringence can then be brought about in some manner such aselectrical or electromagnetic techniques, the nature of thebirefringence so introduced can be studied to determine characteristicsof the material. Upon further consideration the present inventor thencame to the realization that holographic techniques can record both thephase and amplitude, or the polarization and amplitude properties oflight, such as that passing through or reflected off a semiconductormaterial, which can then be reconstructed. Holographic techniquesprovide the ability to examine materials using a wave front that isgreater than, and typically substantially greater than, the physicalfocal point of the wavelength of the light. Further, the presentinventor came to the realization that using field based interferencepatterns detail regarding the structure and operating characteristics ofsemiconductor devices considerably smaller than the physical focal pointof the applied light or applied signal may be determined. Likewise,obtaining such holographic information will enable the development ofother devices, such as for example, lenses, filters, and opticaldevices, which are based on, at least in part, the operatingcharacteristics of semiconductors.

A hologram is created by a coherent light beam being transmitted throughor reflected from an object onto a recording medium, which at the sametime the original beam is also directed onto that recording medium as areference beam. Various characteristics of the resultant transmitted orreflected beam, herein called the “object wave,” are recorded in theresultant interference pattern between the object wave and the referencebeam, i.e., as a hologram. These characteristics can later be observedby illumination of the hologram by that reference beam alone. That is tosay, inasmuch as the phases or polarizations of the reference beam andthe object wave have been recorded in that interference pattern alongwith their intensities, the wave produced by illuminating the hologramwith the reference beam is essentially an exact replica of the objectwave. Those characteristics are in part a consequence of the physicalstructure (i.e., “appearance”) of the illuminated object, hence the waveso observed appears as a three dimensional image of that object. OpticalHolography, Second Edition, by P. Hariharan, 1996 describes some generaloptical holographic techniques, and is incorporated by reference hereinin its totality.

The present inventor also realized that particular semiconductormaterials are generally transparent to light of particular wavelengthsso that the light may freely pass through and reflect back though all ora portion of the semiconductor, or otherwise pass through thesemiconductor, substantially “unaffected” when the semiconductor is notstressed, such as by no applied voltage, not subject to aelectromagnetic (e.g. radio wave) field or signal, magnetic field, x-rayradiation, gravity wave, sub-atomic partical radiation, pressure,temperature, photo-generated carriers, subject to electron- or ion-beamdevices, bioelectric, or chemical. Likewise, when the semiconductormaterial, such as one including an integrated circuit, is stressed byapplying a voltage therein by energizing a circuit fabricated therein,or by applying a different energy level, the same light will reflect orotherwise pass through the semiconductor material, while being affectedby the changes imposed by the applied voltage, thereby resulting in adifferent pattern. The stressed and unstressed states may be recorded asdifferent holographic images. In addition, it is within the scope of thepresent invention to make a comparison between two different stressedstates. The two holographic images may then be compared to one anotherto determine the actual operating characteristics within thesemiconductor material. In addition, by its nature, holographic imagingtechniques record a significant spatial region much larger than a singlewavelength simultaneously which is important for characterizing regionsof the semiconductor material. For example, the operationalcharacteristics of two different regions may be interrelated which isunobtainable using techniques limited to a wavelength in “spot size.”The present inventor's realization that the application of holographictechniques for the testing of semiconductor devices was only after atleast the culmination of the aforementioned realizations.

Of particular interest is the “real time” characterization of operatingcharacteristics of integrated circuits where such birefringence isintroduced by the electro-optic effect, i.e., the imposition of avoltage onto the object (as in the ordinary operation of the integratedcircuit) causes birefringence therein. Birefringence in 3/5 materialsgenerally imposes a rotation of the polarization which is primarily apolarization and/or amplitude shift. Voltage induced changes in siliconmaterials generally imposes a refractive index shift which is primarilya phase and/or amplitude shift. Both of these occurrences aredetectable, such as for example, with a polarized beam, a polarizationrecording hologram, an amplitude hologram, and/or a phase hologram. Inother words, upon application of an electric field the material, such asGaAs or silicon, introduces an anisotropy and the ordinary complexrefractive index n* of the material is decomposed into n_(o)* and n_(e)*components. Another technique applicable to appropriate substrateswhether or not any operational voltages are also applied thereto, liesin utilization of the photo-refraction effect, wherein electromagneticradiation of a required intensity is illuminated onto the substrate, anda birefringence or change in birefringence is then brought about.Inasmuch as semiconductor and like materials are generally characterizedby a wavelength threshold below which photo-refraction will occur, butabove which no photo-refraction takes place, this latter mode ofoperation employs electromagnetic radiation of differing wavelengths,first to bring about a desired photo-refractive effect, and thensecondly to analyze the effect so brought about.

FIG. 4 illustrates a holographic system that presents isopachic images.

FIG. 5A shows a holographic apparatus 200 comprising a laser 202 such asa frequency stabilized infrared filtered He/Ne or NG/YAG lasers with afilter or the like, from which is derived a plane wave of linearlypolarized light 204. The optical path thus defined may optionallyinclude a selected first neutral density filter 206 that will permitconvenient adjustment of the laser power level. The beam 204 from thelaser 202 (or from the filter 206, if used) may then be passed into afirst broad band polarization rotator 208 for purposes of placing theplane of polarization of the laser beam at a desired orientation.Whether or not the polarization rotator 208 is used, the beam may thenbe passed through one or more first waveplates 210 that may optionallybe used to establish a desired degree of ellipticity in the beam. In anycase, the resultant beam then may pass through a first objective lens212 and a first spatial filter 214 to impinge on a first converging lens216 that will then yield an expanded plane wave 218. The converging lens216 may be an achromatic type which is diffraction limited at 1.03 um(the filtered infrared He/Ne wavelength) for optimal 1:1 imaging.Alternatively, first objective lens 212, first spatial filter 214, andfirst converging lens 216 may be incorporated together within a lasercollimator, or in any such similar device.

Plane wave 218 is then incident on a beam splitter 220 that provides tworeference beams: first reference beam 222 a that is incident on testobject (TO) 224, a second reference beam 222 b that will ultimatelyimpinge on recording device (RD) 226. Beam splitter 220 may, forexample, be a pellicle beam splitter. For RD 226, either infraredphotographic film, an infrared sensitive electronic device, such as aninfrared CCD, or an infrared thermoplastic recorder, or any othersimilar device may be used. As a result of first reference beam 222 abeing reflected back from the surface of TO 224, a object beam 228 willpass back onto beam splitter 220 so as to be reflected towards andultimately impinge upon RD 226. Since both a reference beam (secondreference beam 222 b) and an object beam (object beam 228) that derivefrom a common, preferably coherent source (laser 202) are simultaneouslyincident on the recording device 226, the conditions for forming apolarization preserving hologram are present.

Consequently, FIG. 5A is also seen to include a pair of lenses 230, 232,which are meant to provide a generic indication of a beam conditioningelement that may be any one of many types that are well known in theart, and by which the precise degree of focus, convergence ordivergence, or other aspects of the beams that are to impinge on the RD226 can be adjusted It is to be understood that, such beam conditioningis optional. For example, in a fixed system the reference and objectbeams are passed through identical optical components and theirconditioning or lack thereof are the same, i.e., in a firstapproximation the differences between the two beams would derive solelyfrom the effects of the first reference beam 222, having been reflectedfrom the TO 224 so as to yield the object beam 228. More exactly, theforegoing statement assumes that the first and the second referencebeams 222 a, 222 b are identical, which may not be the case because ofdiffering aberrations or the like being present in the beam splitter 220as to the first and the second reference beams 222 a, 222 b. Therefore,the elimination of effects arising from sources other than from objectbeam 228 itself can occur with reference to comparisons of two or moresuch holograms that have been recorded under identical circumstances.

To permit such a procedure, the in line apparatus of FIG. 5A may bemodified with respect to the nature of the recording device in order toproduce additional holograms. Since that modification may itselfintroduce differences in the precise conditions of measurement forreasons other than any optical aberrations in the beam splitter 220, thelenses 230, 232 (or, more exactly, any particular beam conditioningelements that may be employed for such purpose) are to be used tocondition the beams passing therethrough so as to duplicate, in theprocess of recording additional holograms, the conditions of the beamsunder which a first hologram was recorded. For purposes of the presentinvention, and in taking an initial hologram, the TO 224 may be anysuitable to which the characteristics are desired, such as for example,a functional IC on which the surface has been exposed (i.e., potting isnot present) but to which no voltages or other external stimuli havebeen applied, a semiconductor material such as a wafer taken from orexistent within a wafer manufacturing line, a semiconductor wafer takenfrom or existent within a chip manufacturing line at any of variousstages of manufacture (deposition, etching, metallization, etc.) or thelike, the RD 226 may be taken to be any suitable material for recordinga holographic image, such as for example, an infrared photographic filmor an infrared thermoplastic plate onto which the initial hologram isrecorded in the graphic film or thermoplastic plate onto which theinitial hologram is recorded. The recorded infrared film 226subsequently may be viewed by various angles to resolve threedimensional features and details. In this manner, among othertechniques, the voltage pattern of the device may be viewed in a threedimensional manner. Holographic image reconstruction from an voltagepattern or a microwave signal in a device recorded at 1.15 μm may beobtained using a shorter wavelength such as 0.633 μm from recorded filmsor thermoplastics. This is important when multilevel electronic circuitlayout techniques are used so that the voltages may be determined withinthe bulk of the material.

FIG. 5B shows an alternative embodiment, with like elements beingindicated by like numerals but in which, among other changes the RD 226has been replaced by the RD 234, which is a conventional infrared CCDcamera. Since constructive and destructive interference between coherentwaves occur with respect to that electromagnetic radiation itself,without regard to the nature of any device onto which the resultantinterference wave may be recorded, the hologram (which in many cases isconsidered an interferogram) may be recorded by an infrared CCD cameraas well as by infrared photographic film(s) or an infrared thermoplasticrecorder, described later. Consequently, upon extracting the infraredCCD image in the usual manner, one acquires a digital representation ofa hologram as derived from interference between the particular referencewave and object waves that were incident upon the infrared CCD cameraduring the time for which the image was so extracted. Unfortunately, CCDcameras typically have limited spatial and/or temporal resolution.

As to the case in which the TO 224 is a functional but not energized IC,a first hologram can be recorded therefrom using the apparatus as shownin FIG. 5A, i.e., the hologram is recorded onto an infrared recordingdevice, such as for example either onto infrared photographic film orwithin an infrared thermoplastic plate. A second hologram can then bemade of that same TO 224 while either being energized with a DC or an ACvoltage or illuminated with light of a wavelength shorter than thecharacteristic threshold wavelength for the material so that a change isimposed therein. In the case in which the TO 224 is a semiconductorwafer, a first hologram may similarly be recorded and then a secondhologram may be recorded while illuminating the wafer in the manner juststated. In either case, any (birefringence) effects brought about eitherby the electro-optic effect or by the photorefractive effect will thenbe recorded. A comparison of the two holograms, both taken from one orthe other instance of the TO 224, and advantageously by illuminating onehologram through the hologram taken of the other, will isolate suchelectro-optically or photorefractively produced birefringence.Alternatively, the film 226 may be partially exposed to the unstressedimage of the TO 224 and then subsequently partially exposed to thevoltage induced stressed image of the TO 224 (or vice versa) to providea holographic image with interference patterns recorded. The film 226may be subsequently removed and analyzed.

Alternatively, a second hologram can be recorded using the apparatus inthe in line configuration shown in FIG. 5B, e.g., using an infrared CCDcamera 234 as the recording device. However, attempts to compare ahologram taken from CCD camera 234 with another hologram that wasrecorded by any other means introduces inevitable experimental error.However, the comparison of two separately obtained holograms, such asfrom one or more infrared CCD's or other infrared recording devices, iswithin the scope of the present invention. if a first hologram isrecorded using film 226, and then a second hologram is recorded usingCCD camera 234, the two might in principle be compared, e.g., a printmight be made from each of the recording mechanisms (i.e., film 226 andcamera 234), and their differences might then be explored, for example,by using a beam from laser 202. However, making such prints introducesseveral experimental artifacts, including such factors as: (1)differences in the spectral sensitivity of the film and the CCD camera;(2) differences in the mechanics of printing from the two differentmedia, such as a photographic film or from digital data; (3) differencesin the precise experimental configuration at the time the holograms weremade, e.g., replacement of film 226 with camera 234 could not have beenaccomplished with total accuracy; (4) the optical line resolution of theinfrared film and camera based devices. As to the differences arisingfrom a printing process, the photographic film might be developed andscanned, and thereafter treated as digital data, but the first factorinvolving the different spectral sensitivity and the scanning processitself would again present artificial differences between the twoholograms that did not arise from the TO 224. In lieu of the foregoingprocesses, it is more typical to use a first hologram as a “mask”through which the reference beam is transmitted while making the secondhologram (thus showing differences), but even in this case the film mayneed to be removed, developed and then replaced, so the placement errorsjust mentioned may still be present.

One of the sources of error as just noted is removed when recording afirst hologram onto an infrared thermoplastic plate. For example, if aholographic recorder is used in conjunction with an installedthermoplastic plate, after exposure the thermoplastic plate is developedin situ, i.e., the plate is not removed from the optical path for suchpurposes. The error brought about by removing the recording medium fordevelopment and then replacing that medium back into the experimentalapparatus is thus eliminated. Also, holographic image reconstructionfrom an voltage pattern or a microwave signal in the thermplastic devicerecorded at a wavelength of 1.15 μm may be obtained using a shorterwavelength such as 0.633 μm by increasing the image by a factor of two.Also, differences in the spectral sensitivity and optical resolution(lines per unit of length) of the thermoplastic plate and CCD camera 234remain as a significant source of a experimental error.

A CCD camera 234 may be used as the sole recording device, whereby thefirst and indeed a multiplicity of subsequent holograms can be recorded.If the recording rate of the CCD camera 234 is slower than the rates ofoperation of an IC itself, timed optical pulses may be used for samplingthe device under test. An additional advantage in using only the CCDcamera for recording holograms is that the “reference” hologram, i.e.,the hologram recorded from the TO 224 (either as an IC or as asemiconductor wafer) at a time that no voltages orbirefringence-inducing laser light was applied thereto, will be recordeddigitally as well, and comparisons between the reference and subsequentholograms can be made by means other than within the experimentalapparatus itself, i.e., by ordinary digital signal processing (DSP).

For the purpose of processing such a data stream, FIG. 5B also includesan analyzer 236 connected to the CCD camera 234, and also a monitor 238connected to analyzer 236 Inasmuch as the laser source in the presentembodiment is preferably a pulsed CW NG/YAG laser with a filter, thedata to be analyzed may be generated by triggering the recording of CCDimages in synchrony with the imposition of particular voltage data ontothe TO 224, which may be an IC or possibly an entire printed circuit. Asnoted previously, the Springer patent describes the use of a digital“kernel” comprising a predetermined test program together with thedigital data to be employed by that program, both of which are stored inROM. The Springer apparatus then uses voltage probes and the likeapplied to various circuit nodes to test circuit performance in a“manual” fashion; the present invention, of course, centers on an“automatic” process of testing an entire IC, circuit board or, as willbe shown below, a semiconductor wafer at any desired stage ofmanufacture. FIG. 5B thus shows a device driver 240 which connects tothe TO 224 through a bus 242 and carries voltage data 244 thereto, whilea trigger line 246 which connects from the device driver 240 to CCDcamera 234 conveys a trigger signal 248 thereto, the relative timing bywhich voltage data 244 and a trigger signal 248 are so transmitted beingestablished such that CCD camera 234 records one or more images at atime that the voltage data 244 have been applied to the TO 224. Withinthe limits of the operational characteristics of the CCD camera 234, thedynamical processes by which the voltage data 244 have particulareffects within the TO 224 (e.g., the turning on or off of a transistor,a voltage pulse propagating down a bus, etc.) can be traced bytransmitting the trigger signals 248 to the CCD camera 234 at somemultiple of the frequency at which the voltage signals 244 are sent tothe TO 224, so as to evaluate such parameters as transistor rise or falltime. It is to be understood that other recording devices 234 maylikewise be used.

In order to illustrate the uses of the aforesaid components and those tobe describe hereinafter, it is appropriate to outline in greater detailthe characteristics of an exemplary representative TO 224, which forpurposes of illustration to is taken be a wafer of GaAs that has beenindustrially prepared as a substrate for subsequent IC manufacture. GaAsis a direct-gap (but nearly indirect-gap) semiconductor having the zincblende structure with a lattice constant of 0.5653 nm, i.e., it has theface-centered cubic structure but without inversion symmetry, and thusbelongs to the point group Td. The static dielectric constant of GaAs is12, and the exciton binding energy E_(B) is 4.2 meV.

T_(d) symmetry defines an isotropic material that at any selectedwavelength exhibits but a single refractive index n. In order to observebirefringent effects in GaAs, therefore, it is necessary to induce thesame, either by the electro-optic effect or by the photorefractiveeffect. The manner in which such effects can be measured mostproductively can be related to the manner in the birefringence soproduced relates to the GaAs crystal structure. That is, changes in theoptical properties of the GaAs crystal that render it birefringent maybe treated in terms of that crystal no longer being cubic, i.e., anycrystal that exhibits birefringence should have one or more unique opticaxes and hence, at least with respect to optical refraction, no longerexhibits cubic symmetry.

For purposes of future reference, the standard definitions of the planesin a cubic crystal are shown FIG. 6A, wherein a face-centered cubiccrystal is superimposed on a coordinate system with the origin at thecenter of the crystal, the (100) plane cuts the x axis, the (010) planecuts they axis, and the (001) plane cuts the z axis. The GaAs wafer iscleaved along one such plane, but since these planes are physicallyidentical, for convenience the surface plane of a GaAs crystal (and ofthe ICs formed therefrom) is usually referred to as the (001) plane, andthe normal to that plane is the z axis. The x and y axes thus lieorthogonally within the (001) plane, and one issue that arises in thecourse of manufacturing IC's from such a wafer, and which can beresolved using the present invention, lies in determining the locationof those x and y axes so as to permit the marking of the wafer toindicate that orientation for purposes of later IC fabrication.

FIG. 6B shows the refractive index indicatrix for an isotropic (i.e.,cubic) crystal such as silicon based materials, which is seen to havethe form of a sphere, and from which it can be seen that the refractiveindex of the material has the same value in all directions (i.e., anyline through the center of the indicatrix has the same radius as anyother line through that center). Upon application of an externalelectric field, or of light having sufficient photon energy to causephotorefraction, the material will become birefringent. For purposes ofdiscussion, the refractive index indicatrix of both positive andnegative uniaxial crystals that have two refractive indexes, i.e., forthe ordinary (n_(o)) and the extraordinary ray (n_(e)), are shown inFIGS. 6C and 6D. In effect, birefringence induced by either of theaforesaid methods converts the crystal structure into one that is nolonger of the cubic class. It thus becomes necessary to identify theclasses that may so be formed, and the means for forming them. For thatpurpose, FIG. 6E shows the indicatrix and resultant wave propagation fora generalized wave normal, which may be taken as a light wave that isincident on the crystal at some arbitrary angle.

The symmetry aspects of applying external forces to crystals have beendiscussed by J. F. Nye in Physical Properties of Crystals (ClarendonPress, Oxford, 1972), pp. 235-259, and especially pp. 245-246. Aprincipal characteristic of such force applications is that a force thathas one or more symmetry elements in common with those of the crystal towhich the force is applied will be effective, while any symmetryelements of that force that is do not likewise characterize the crystalwill have no effect thereon. At first glance, this would not seem to beparticularly important as to GaAs which, being a cubic crystal, has themaximum number of symmetry elements. However, inasmuch as the result ofapplying a force having a particular symmetry element is exhibited byway of altered features of the crystal that share that particularsymmetry element, the precise nature of the effect brought about willdepend upon which symmetry elements characterized the applied force. Forexample, while any voltage of sufficient magnitude will causebirefringence in a GaAs crystal, but precisely what kind ofbirefringence that will be and how it would appear if examinedholographically will depend upon the orientation of that electric fieldrelative to the crystal axes.

An electric field that is normal to the exposed (001) face of a GaAscrystal would have the effect of defining the corresponding z axis asthe optic axis. An electric field that is tangential to that (001) facewill define an optic axis lying somewhere in that face, i.e., eithercoincident with one or the other of the x and y axes or lying at someangle thereto. The indicatrix for the resultant uniaxial “structure,”which will then determine the directions of propagation of thetransmitted rays that would result from imposing an incident beam oflight thereon, will have a corresponding orientation relative to the xand y axes. Because of the induced birefringence, there will be tworays, linearly polarized at right angles to each other, that transmitthrough the uniaxial crystal so formed, and by conservation principlesthe corresponding rays that reflect therefrom will likewise be sopolarized. As will be seen below, this combination of events lendsitself to direct determination by the present invention, since theapparatus that embodies the invention preserves not only the amplitudeand phase of the reflected radiation as in a normal hologram, but alsothe polarization.

FIG. 7 depicts an in line holographic apparatus 300 which analyzessemi-conductor materials to which no voltages have been applied, butonto which a laser beam can be transmitted so as to bring about inducedbirefringence, as the source of holographically detectable refractiveindex or polarization changes. Holographic apparatus 300 differs fromholographic apparatus 200 in including therein a first activating lasermodule 350, which centers on the use of a higher energy (and preferablyof a highier photovoltaic generating power) laser 352, which may, e.g.,comprise an argon ion laser of a ultraviolet laser. Laser beam 354emitting therefrom transmits through a second objective lens 356, asecond spatial filter 358 and a second divergent lens 360 to a thebroader beam 362 that is then incident on the TO 224. Deriving as itdoes from an argon ion laser, the laser beam 354 may have a wavelengthof 458 nm, corresponding to a photon energy of 6.946 ev, which willsuffice for the generation of refractive index changes therein formaterials such as GaAs that have a lower threshold energy forphotorefraction. For that purpose, the pulse generator 264 transmits thepulse 266 over a line 368 so as to generate a pulse of light from thelaser 352, and one or more appropriately timed triggers 370 are sentover a trigger line 372 to the CCD camera 234 (or other recordingdevice) to acquire one or more images of the TO 224 under the condition,as just indicated, that the laser beam 362 is incident thereon. Thisprocess may be referred to as stroboscopic holographic interferometrywhere a hologram of a vibrating object is recorded using a sequence ofpulses that are triggered at times delta t1 and delta t2 during thevibration cycle. The hologram is equivalent to a double-exposurehologram recorded while the object was in these two states ofdeformation, and the fringes have unit visibility irrespective of thevibrating amplitude. The phase of the vibration can be determined from aseries of holograms made with different values of delta t2, keepingdelta t1 fixed; alternatively, real-time observations can be made. Inessence, the pulses are preferably timed to the operation of the testobject.

With regard to another aspect of holographic apparatus 300 (and of allvariations thereof described herein), the ability of holographicapparatus 300 and variations thereof to accomplish measurements of anentire wafer (or portion thereof) or the like at once provides yetanother advantage. That is, region 274 shown in FIG. 7, which containsonly the TO 224, interacts with the rest of holographic apparatus 300only by way of light beams, i.e., by the first reference beam 222 a, theobject beam 228, and the laser-beam 362. Region 274 may then constitutea clean room in which the manufacture of wafers, or the ICs to bederived therefrom, is actually carried out. Holographic apparatus 300and variations thereof thus make possible a complete regime of qualitycontrol in IC manufacture, at every stage from the initial wafer to thepoint at which the wafer is diced into individual ICs. In the discussionwhich follows, it may then be assumed that the TO 224 under discussionis located within such a clean room, and the testing apparatus islocated in a separate room, connected thereto only through a transparentmedium (e.g., glass) which precludes passage of contaminants but yetallows passage of the first reference beam 222 a, the object beam 228,and the laser beam 262 or variations thereof. As in the case of otherexperimental conditions that might affect the precise nature of theholograms obtained, any variations therein that derive from the presenceof such a medium within the optical path will be eliminated in theprocess of comparing holograms that were taken under identicalconditions.

Different embodiments of the invention as described also differ in thenature of the elements used to transmit appropriately structured laserbeams onto the TO 224, and for that purpose the second laser module 280is shown in FIG. 8, which includes elements additional to those of thefirst laser module 350 of FIG. 7. Specifically, the second laser module280 includes in sequence a laser 282 (that may be identical to previouslaser 352) that yields a laser beam 284 (that may be identical to laserbeam 204), a second neutral density filter 286 (that may be identical tothe first neutral density filter 206 as used elsewhere in holographicapparatus 300), a second polarization rotator 288 (that may be identicalto the first polarization rotator 208 as used elsewhere in holographicapparatus 300), one or more second waveplates 290 (that may be identicalto the first waveplates 210 as is used elsewhere in holographicapparatus 300), a third objective lens 292 (that may be identical toeither of the first objective lens 212 as is used elsewhere inholographic apparatus 300 or to the second objective lens 356 as used inthe first laser module 350), a second spatial filter 294 (that may beidentical to the first spatial filter 214 as is used elsewhere inholographic apparatus 300 or to a second spatial filter 358 as used infirst laser module 250), and finally a third divergent lens 296 (thatmay be identical to first divergent lens 216 as is used elsewhere inholographic apparatus 300 or to the second divergent lens 360 as used inthe first laser module 350), the uses of which then yield a broadenedlaser beam 298 that will be incident on the TO 224 in the same manner asis the laser beam 362 from the first laser module 350. The additionalelements of second laser module 280, i.e., the second neutral densityfilter 286, the second polarization rotator 288, and the secondwaveplates may be used in the same manner as are the corresponding firstneutral density filter 206, first polarization rotator 208, and firstwaveplates 210, namely, to adjust the power level of laser beam 284,define the plane of polarization of laser beam 284, and establish adesired degree of ellipticity of laser beam 284, each of whichadjustments will of course have corresponding effects on laser beam 298.

Including the additional laser module 280 or 350 provides additionalbenefits. If the laser module provides a small illumination point at aphoto-generating wavelength, such as for example ultraviolet, thencircuits may be triggered. By using a broad beam substantial portions ofthe test object may be illuminated which helps identify test objectcharacteristics, such as for example fault locations. Also, if the lasermodule provides a non-invasive beam it may be used to characterizeindividual portions of the test object.

That is, in apparatus 300 of FIG. 7, wherein the TO 224 is taken for themoment to be a GaAs wafer having the (001) face thereof exposed toreference beam 222 a, the precise state of polarization of the planewave 218 can be predetermined by virtue of the first polarizationrotator 208 and the first waveplates 210. Such a predetermination is notreadily-made with respect to plane wave 362 of FIG. 7, but may bereadily made with respect to the plane wave 298 of FIG. 8 by virtue ofsecond polarization rotator 288 and second waveplates 290. Inasmuch asthe electric vector E of a light beam lies transverse to the directionof propagation, and light beam 298 of FIG. 8 will be incident on the TO224 in essentially the same manner as is beam 362 of FIG. 7, the Evector of the beam 298 when linearly polarized will lie in a planedefined by the z axis of the TO 224 and a second axis that liessomewhere in the x, y plane. Similarly, for silicon the change in therefractive index will produce a change in phase, which may bedetermined.

The fabrication of integrated circuits typically uses structures, suchas metal lines and ground planes, that are not generally transmissive,unless sufficiently thin. Using a transmissive holographic techniquerequires the light to pass through the test device. On extremely highdensity devices the feature density may be too high to permit sufficienttransmission. The presence of a ground plane may also restricttransmission. To accommodate structures that are not sufficientlytransmissive a reflection technique may be employed. Transmissive andreflective techniques typically require different holographic systems.Referring to FIG. 9, a set of potential transmission and reflectionstructures are shown suitable for transmissive and reflectiveholographic techniques.

Unfortunately, the in-line holographic techniques result in the virtualimage and the coherent source being viewed in-line and are superimposedupon one another making analysis difficult. The present inventorrealized that an off-axis technique results in two images that areseparated, as in the reference beam. This results in a clearerunobscured image that may be readily used, such as for interferometry.One example of an off axis technique is shown in FIG. 10 where theobject and the reference beams are imaged on the recording medium, usinga transmissive technique. For example, when the test device 224 isunstressed a holographic image is recorded on the recording device 226.Then the test device 224 is stressed in some manner or otherwiseactivated and another holographic image is superimposed on the recordingdevice 226. Preferably a light valve 347 is used to adjust the referencebeam intensity to match the object beam intensity using a set ofdetectors 349. The result is an interference pattern on the recordingdevice 226. Alternatively, two (or more) holographic image may beseparately obtained and analyzed. As in all other embodiments, twodifferently stressed states may be used, if desired.

Another example of an off axis transmissive beam technique is shown inFIG. 11 where the object and the reference beams are imaged on therecording medium using a reflective technique. For example, when thetest device 224 is unstressed a holographic image is recorded on therecording device 226. Then the test device 224 is stressed in somemanner or otherwise activated and another holographic image issuperimposed on the recording device 226. The result is an interferencepattern on the recording device 226. Alternatively, two (or more)holographic image may be separately obtained and analyzed.

Referring to FIG. 12, illustrates a transmissive based system thatimages at a position beyond the recording device 226. The effect of thesystem of FIG. 12 is a reduction in the noise.

The present inventor came to the further realization thatinterferometric testing, as described by Weingarten et al. in a paperentitled, “Picosecond Optical Sampling of GaAs Integrated Circuits”,IEEE Journal of Quantum Electronics, Vol. 24, No. 2, February 1988, islimited to the rules of conventional microscopy. Such limitationsinclude the inability to resolve particles with a limited depth offield. In addition, the system described by Weingarten et al. requiresaccurate alignment of the polarization wave plates in order to interferethe light, which is difficult at best to achieve. Referring to FIG. 13,an improved interferometric tester includes an infrared laser 500 thatprovides an infrared beam or pulse that passes through a polarizer 501and a beam splitter 502. A quarter wave plate 504 provides a rotation ofthe polarization plane to align the beam with the crystal face or testobject. A half wave plate 506 rotates the beam so that the returningbeam is 180 degrees out of phase with the incident beam. A zero waveplate 507 helps to correct for spurious reflections may be included, ifdesired. A microscopic lense 508 expands the beam and the beam strikesthe test object 510. A pulse generator 512 provides energization to thetest object. Alternatively, the test object may be energized with anyother suitable technique, such as a photo-generating beam that generatesa voltage or current therein. The beam is then demagnitized by themicroscopic lense 508 and deflected by the beam splitter 502 to beam540. A microscopic lense 514 expands the beam which is then incident ona sensor 516, such as for example an infrared CCD camera. This providesa wide range field of view substantially greater than a singlewavelength focal point of the light on the test object. Unfortunately,the sensitivity of the system is limited to the optical resolution ofthe optical components, the wavelength sensitivity of the CCD camera,the minimum detectable photo-currents of the CCD camera, and the densityof the pixels of the camera. In addition, it is difficult at best toresolve device structures within the test object that are less than thewavelength of the laser beam. For example, infrared light may be 1.03microns while the device structures of the test object may be 0.14microns.

The present inventor came to the realization that if the CCD cameraresolution and sensitivity to photo-generated voltages could be enhancedtogether with increasing the ability of the CCD camera to resolveindividual pixels, then the system would not be directly limited to thephysical attributes of the CCD camera. To achieve such enhancements thepresent inventor determined that using both holographic microscopyand/or holographic interferometry of the voltage displacement contoursor interferometric fringes of the CCD camera may be used tosubstantially enhance the resolution. The optical image on the CCDproduces a varying electric field on the pixels and associatedstructures which in result modulates the refractive index of thesemiconductor crystals through the electro-optic effect or rotates thepolarization of the incident beam. If the incident light distributionlight changes the crystal in a manner that is different than wasobtained by the CCD at rest, then a new charge pattern and thus amodified image may be obtained. A second sensor that is sensitive to adifferent wavelength of light than the first sensor (CCD) is used tosense the voltage patterns imposed on the first sensor. The secondsensor may then observe the fringes in the voltage patterns of the firstsensor representative of the variations of voltage as a result ofstressing the test device. Inspection of the fringe pattern givesconsiderable amount of information about the stressed test device.Referring to FIG. 14A, initially an in-line holographic reflectionprobing system to illustrate the measurement of a CCD with a CCD isshown, may include a polarized beam preferably having a wavelength of1.03 microns, or 1.3 microns or greater. The beam 560 passes through acamera lense assembly 564 to make the size of the beam 562 similar tothe CCD camera pixel array 566. The beam 562 is reflected to the beamsplitter 568 and reflected. The beam 570 is incident on a CCD 572. TheCCD 572 is sensitive to the wavelength of the beam 560/562/570. A secondbeam 548 of a different wavelength of light than the first beam 562,such as for example 700 nm, creates a photo-current in the CCD 566. Thebeam 562 is non-invasive to the CCD 566 thereby not generatingphoto-voltages therein. The beam 562/570 in essence probes the inducedvoltages of the CCD 566. The beam 562 preferably is incident on the backof the CCD 566, while the beam 548 is incident on the front of the CCD548. The CCD 566 is preferably sensitive to only visible light. The CCD572 records the infrared image as an infrared hologram. The CCD 572senses the voltage induced changes in the refractive index and/orpolarization shifts in the optical properties of CCD 566. Changes in theCCD 566 change the phase and/or polarization of light incident to theCCD 572 resulting in detectable changes.

The inventor then realized, holographic microscopy when combined withconventional microscopy offers much greater usefulness in recording bothphase and amplitude than is obtainable with conventional optical lenselements and devices. Likewise, optical holography offers the means toeliminate cumbersome optics, beam polarizers, and filters and permit insitu placement of more sensitive optics within the ergometric dimensionsof conventional semiconductor packaging.

Holographic measurements of dynamic voltage patterns in semiconductorsand freecarrier, devices distributed throughout an appreciable volumeand area are not possible with a conventional optical microscopicsystem. This is because a conventional microscope which can resolveparticles of diameter d has a limited depth of field as shown in thefollowing equation:

Δz≈d ²/2λ

Holography permits storing a high resolution, three-dimensional image ofthe whole field at any instant. The sationary image reconconstructed bythe hologram can be examined subsequently in detail, throughout itsvolume by a conventional IR microscope.

In-line holography can be used for such examinations whenever asufficient amount (>80%) is directly transmitted to serve as a referncebeam. This premites very simple optical system as shown in FIGS. 14a, 14b, and 14 c. However a a distinction must be made between such anin-line hologram of a semiconductor integrated circuit and a Garborhologram. Because of the small diameter d of the microchip's devicelithography, the distance z of the recording plane from themicroelectronic device(s) under test easily satisfies the far fieldcondition:

z>>4λ/d

where the diffracted field field due to the voltage induced refractiveindex or polarization shifts is its Fraunhofer diffraction pattern.

To give a satisfactory reconstructed image of a semiconductor deviceunder test, the hologram should record the central maximum and at leastsidelobe of its diffraction pattern. This would correspond to IR wavestravelling at a maximum angle

θ_(max)=4λ/d

to the directly transmitted wave, and hence, to a maximum fringefrequency of 4/d, which is independent of the values of λ and z.Accordingly, for a device under test with 10 μm lithography, therecording material should have a modulation transfer function whichextends beyond 400 mm⁻¹.

Recording devices and films with lower resolution, which are faster andeasily employed, can be used effectively if a hologram is recorded of amagnified image of the device under test. For this, the imaged area ofthe device under test is imaged near the holographic plane, as shown inFIGS. 14a, 14 b, and 14 c, with a telescopic system having amagnification between 100 and 1,000. This leaves the reference beamparallel and gives constant magnification over the whole depth of thefield.

The inventor then realized from this material that the depth of thefield is also limited by the dimensions of the recording material.Examination of the previvious expression for calculating the maximumangle, it follws that x, the half-wigth of the hologram, must be greaterthan 4zλ/d. Hence, the maximum depth of field over which the requiredresolution can be maintained is given by the relation

Δz _(max) =xd/4λ

Determining film or recording medium resolution requirments for off-axismultiple beam holographic recording setups.

Holographic interferometry of phase objects provides a very simpleapproach to In-line holography when a sufficent amount of light isdirectly transmitted throught the device under test serves as areference beam. Even in applications such as flow visualization and heattransfer studies, where conventional interfermetry has been used formany years, holographic interferometry has practical advantages.

In this first instance, mirrors and windows of relatively low opticalquality can be used. [e.g., the 1 cm holographic plate is much largerthan the 10 μm object. Since the phase errors due to these contributeequally to both interfering wavefronts, they cancel out, and only theeffects changes in the optical path are seen. However, a significantadvantage is the possibility of incorporating a diffusing screen (aground glass plate) in the interferometer. This gives an interferencepattern that is localized near the phase object, and can be viewed andphotographed over a range of angles. This makes it possible to studythree-dimensional refractive index distributions.

If the refractive index gradients in the test section are assumed to besmall, so that rays propagate through it along straight lines parallelto the z axis, φ(x,y) the phase difference at any point in theinterference pattern is given by the relation:

φ(x,y)=ko∫[n(x,y,z)−no]dz  (64)

where no is the refractive index of the medium in the test section inits initial unpreturbed state and n(x, y, z) is the final refractiveindex distribution.

The simplest case is that of a two-dimensional phase object with novariation of refractive index in the z direction. In this case therefractive index distribution can be calculated directly from (64) Thisis a valid approximation to many practical situations.

Another case which lends to analytic treatment is that of a refractiveindex distribution f(r) which is radially symmetric about an axis normalto the line of sight (for convenience say, the y axis).

For a ray traveling in the z direction at a distance x from the center,we then have: $\begin{matrix}{{d \cdot z} = {\left( {r^{2} - x^{2}} \right)^{\frac{1}{2}} \cdot r \cdot d \cdot r}} & (65)\end{matrix}$

so that (64) becomes: $\begin{matrix}{{\varphi \left( {x,y} \right)} = {2 \cdot {\int_{x}^{\infty}{{f \cdot (r) \cdot \left( {r^{2} \cdot x^{2}} \right)^{\frac{1}{2}}}\quad {r^{2}}}}}} & (66)\end{matrix}$

This is the Abel transform of f(r), and it can be inverted to find f(r).

The evaluation of an asymmetric refractive index distribution f(r,θ) ismuch more difficult and is possible only by recording a large number ofinterfergrrams from different directions. The problem becomes even morecomplicated when the effects of ray curvature due to refraction cannotbe neglected.

Holographic interferometry has been found particularly useful in highenergy plasma diagnostics. Since, unlike a neutral gas, a plasma ishighly dispersive, measurements of the refractive index distribution at[different] two wavelengths make it possible to determine the electrondensity directly. This has shown that “the interference fringes arecontours of constant dispersion and, hence of constant electrondensity.” Both these approaches while demonstrating plasma diagnostics,are limited in their utility because of the inability of the twoholograms to provide real-time interferometry, as needed.

When using Real-Time holographic interferometry techniques, the hologramis replaced, after processing, in exactly the same position in which itwas recorded. When it is illuminated with the original reference beam,the virtual image coincides with the object. If, however, the shape ofthe object changes very slightly, two sets of light waves reach theobserver, one being the reconstructed wave (corresponding to the objectbefore the change) and the other directly transmitted wave form itspresent state (e.g., changes in the index of refraction will alter thereflected optical path).

The two wave amplitudes will add at the points where the difference inoptical paths is zero or a whole number of wavelengths, and cancel atsome other points in between. As a result, an observer, photodetector ordevice viewing the reconstructed image sees it covered with interferencefringes, which is a contour map of the changes in shape of the object.These changes can be observed in real-time.

Considering an off-axis holographic recording system, the intensity atthe photographic plate when the hologram is recorded is:

I(x,y)=(|r·(x,y)+o·(x,y)|)²   (67)

where r(x,y) is the complex amplitude due to the reference beam ando(x,y)=|(x,y)|exp[−1φ(x,y)] is the complex amplitude due to the objectin its normal state.

Assuming that the amplitude transmittance of the photographic plateafter processing is linearly related to the exposure, the amplitudetransittance of the hologram is:

τ(x,y)=τ_(o) +β·T·I(x,y)   (68)

An alternative to FIGS. 4,5 a, 5 b, 7,10,11,12,13,14A is a two-beammicroscopic holographic reflection probing system shown in FIG. 14c, toillustrate the measurement of TO 224 or a CCD with a CCD is shown, mayinclude a polarized beam preferably having a wavelength of 1.03 micronsor 1.3 microns. The beam 760 passes through a camera lense assembly 764to make the size of the beam 562 similar to the CCD camera pixel array766. The beam 762 is reflected to the beam splitter 768 and reflected.The beam 770 is incident on a CCD 772. The CCD 772 is sensitive to thewavelength of the beam 760/762/770. A second beam 748 of a differentwavelength of light than the first beam 762 such as for example 700 nm,creates a photo-current in the CCD 566 or TO 224. The beam 762 isnon-invasive to the CCD 766 thereby not generating photo-voltagestherein. The beam 762/770 in essence probes the induced voltages of theCCD 766. The beam 762 preferably is incident on the back of the CCD 766,while the beam 748 is incident on the front of the CCD 748. The CCD 766is preferably sensitive to only visible light. The CCD 772 records theinfrared image as an infrared hologram. The CCD 772 senses the voltageinduced changes in the refractive index and/or polarization shifts inthe optical properties of CCD 766. Changes in the CCD 766 change thephase and/or polarization of light incident to the CCD 772 resulting indetectable changes.

A two-beam microscopic holographic transmission probing system shown inFIG. 14C, to illustrate the measurement of TO 224 or a CCD with a CCD isshown, may include a polarized beam preferably having a wavelength of1.03 microns or 1.3 microns. The beam 860 passes through a camera lenseassembly 864 to make the size of the beam 862 similar to the CCD camerapixel array 866. The beam 862 is reflected to the beam splitter 868 andreflected. The beam 870 is incident on a CCD 772. The CCD 872 issensitive to the wavelength of the beam 860/862/870. A second beam 848of a different wavelength of light than the first beam 862, such as forexample 700 nm, creates a photo-current in the CCD 566 or TO 224. Thebeam 862 is non-invasive to the CCD 866 thereby not generatingphoto-voltages therein. The beam 862/870 in essence probes the inducedvoltages of the CCD 866. The beam 862 preferably is incident on the backof the CCD 866, while the beam 848 is incident on the front of the CCD848. The CCD 866 is preferably sensitive to only visible light. The CCD872 records the infrared image as an infrared hologram. The CCD 872senses the voltage induced changes in the refractive index and/orpolarization shifts in the optical properties of CCD 866. Changes in theCCD 866 change the phase and/or polarization of light incident to theCCD 872 or TO 224 resulting in detectable changes.

The arrangements shown in FIGS. 14A/14B/14C can also be employed toenhance the senistivity of other voltage measurement and electronicdevices shown in FIG. 15A and FIG. 15B which are fabericated insubstrate with or integrated into TO 224 or CCD 872. Examination of thetargeted Joesphson junction device's diameters in solid state electronicinterferometer are 4 μm² and 7 μm² respectively, with a line width of 2μm on a Gate Delay Joesphson OR Gate with Modified Variable ThresholdLogic. FIG. 15A shows the faberication layout and the targetedholographic examination area of the Joesphson device. Where multipleJoesphson devices are employed in an electronic device or substrate,multiple target-specific probing beams are utilized. These arrangementscan also be employed for the enhanced measurement resolution forelctronic devices detecting the presence of the following forces actingupon semiconductor and free-metal devices; electromagnetic (e.g. radiowave) field or signal, magnetic field, x-ray radiation, gravity wave,sub-atomic partical radiation, pressure, temperature, photo-generatedcarriers, incident electron- or ion-beams, bioelectric or chemical.

By analyzing the position of the holographic fringes and the shiftsreveals information as to the nature of the voltages in the CCD orwithin the Joesphson devices. The result is that the holographic fringesprovide a high sensitivity to changes in voltage of the test device.Also the fringes increase the density of the sampling points of the CCDnot directly provided by the pixels themselves. Analysis of the datafrom the CCD may provide information regarding the structure of the testdevice substantially smaller than the wavelength of the light used.

Referring to FIG. 15A, a system that includes primarily CCD's isillustrated. The magnified sensitivity of the secondary CCD to theprimary CCD premites greater field sensitivity and resolution to employholographic Speckle inferometry techniques and devices to the test andevaluation of TO 224.

Factors limiting the resolution holographic lithography and photographyare not so much limited by the quality optics as in conventualphotography and lithographic techniques but, upon the coherent nature ofthe hologram's interfering coherent beams or wavefronts upon therecording medium.

For infrared testing it may be desirable to include an infraredsensitive photo-conductor as the holographic recording medium. Generallysuch an infrared sensitive photo-conductor generates a photo-currentwhen exposed to infrared light. Such a photo-conductor may be, forexample, silicon (for creating photogenerated voltages up to shortinfrared wavelengths of 1.1 μm), Lead Selenide, Indium Arsenide, GaAs,PbTe, GaAs doped with zinc, and Lead Sulfide. Accordingly, thephoto-current generated in the infrared sensitive photo-conductor isgenerally less than one electron volt. The photo-current then performsthe step of reducing the surface potential, as in standard visible lightphoto-thermoplastics for in-line and off-axis holographic techniques.

The unique capabilities of holographic interferometry are due to thefact that holography permits storing a wave front for reconstruction ata later time and are suitable for electrical measurement. Wavefrontswhich were originally separated in time or space or even wavefronts ofdifferent wavelengths can be compared by holographic interferometry. Itis possible to observe holographic interferometry in real time. Afterprocessing the first hologram, leaving the holographic material in placeor replacing it, when it is illuminated with the original referencebeam, it reconstructs the object wave, and the virtual image coincideswith the object. If, however, the characteristics of the object changesvery slightly, two sets of light waves reach the observer, one being thereconstructed wave (corresponding to the object before the change) andthe other the directly transmitted wave from the object in its presentstate. The two wave amplitudes add the points where the difference inoptical paths is zero, or a whole number of wavelengths, and cancel atsome other points in between. As a result, an observer viewing thereconstructed image sees it covered with a pattern of interferencefringes, which is a contour map of the changes in shape of the object.These changes can be observed in real time. In addition, by viewing atmultiple angles, a three dimensional model may be developed of theobject. One general limitation of real-time holographic interferometryis that while the light diffracted by the hologram is linearlypolarized, the light scattered by a diffusely reflecting object islargely depolarized, resulting in a significant drop in the visibilityof the fringes. To avoid this, it is helpful to use a polarizer whenviewing or photographing the fringes.

In double exposure holographic interferometry which may be used to testelectronic devices, interference takes place between the wavefrontsreconstructed by two holograms of the object recorded on the samephotographic plate. Typically, the first exposure is made with theobject in its initial, unstressed condition, and the second is made witha stress applied to the object. When the processed hologram isilluminated with the original reference beam, it reconstructs twoimages, one corresponding to the object in its unstressed state, and theother corresponding to the stressed object. The resulting interferencepattern reveals the changes in shape of the object between the twoexposures. Double exposure holographic interferometry has an advantageover real-time holographic interferometry, because the two interferingwaves are always reconstructed in exact register. Distortions of theemulsion affect both images equally, and no special care need be takenin illuminating the hologram when viewing the image. In addition, sincethe two diffracted wavefronts are similarly polarized and have almostthe same amplitude, the visibility of the fringes is good. However,double-exposure holographic interferometry has certain limitations. Thefirst of which is that where the object (or object's refractive index)has not moved between the exposures, the reconstructed waves, both ofwhich have experienced the same phase-shift, add to give a bright imageof the object. As a result it is difficult to observe smalldisplacements or changes in the material's index of refractive orpolarization state. A dark field, and much higher sensitivity, can beobtained by holographic subtraction, which merely involves shifting thephase of the reference beam by 180 degrees between the two exposures. Analternative method, which also helps to resolve ambiguities in the senseof changes, is to shift the phase of the reference beam by 90 degreesbetween the two exposures, or, better, to introduce a very small tilt inthe wave-front illuminating the object between the two exposures. In thelatter technique, equally spaced reference fringes are obtained, whosechange is modulated by the phase shifts being studied. Anotherlimitation of the double-exposure technique is that information on theintermediate states of the object is lost. This problem can be overcometo some extent by multiplexing techniques using spatial division of thehologram. In the latter procedure, a series of masks are used in whichthe apertures overlap in a systematic fashion, and a sequence ofholograms is recorded at different stages of loading. The images canthen be reconstructed, two at a time, so that interference patternsbetween any two images can be studied. An alternative is to usethermoplastic recording material, by which real-time fringes can beobserved and the fringe pattern subsequently frozen to give a permanentholographic record.

Control of the fringes, to compensate for rigid body motion andeliminate ambiguities in interpretation, is not normally possible with adoubly exposed hologram. However, it is possible with two hologramsrecorded with different angularly separated reference waves. Theseholograms may be either on the same plate or different plates.

The testing of electronic devices in semiconductor materials maylikewise be tested using stroboscopic holographic interferometry where aholograph of a stressed test object is recorded using a sequence oflight pulses that are triggered at different times during the stressingof the test object.

A surface relief hologram can be recorded in a thin layer ofthermoplastic which is combined with a photo-conductor and charged to ahigh voltage. On exposure, a spatially varying electrostatic field iscreated. The thermoplastic is then heated so that it becomes soft enoughto be deformed by the field and, finally, cooled to fix the pattern ofdeformation. Such materially have a reasonably high sensitivity over thewhole visible spectrum and yield a thin phase hologram with fairly highdiffraction efficiency. In addition, they have the advantage that theydo not require wet processing. If a glass substrate is used, thehologram can be erased and the material re-used a number of times. Onestructure of a photo-thermoplastic is a multi-layer structure consistingof a substrate (e.g., glass, Kodar, or Mylar) coated with a thin,transparent, conducting layer, an infrared sensitive photo-conductor,and a thermoplastic. Referring to FIG. 16, the film is initiallysensitized by applying a uniform electric charge to the top surfaceusing a corona device that moves over the surface at a constant distancefrom it and sprays positive ions on to it. As a result a uniformnegative charge is induced on the conductive coating on the substrate.Next, the film is exposed, and the charge carriers are produced in thephoto-conductor wherever light is incident on it. These charge carriersmigrate to the two oppositely charged surfaces and neutralize part ofthe charge deposited there during the sensitizing step. This reduces thesurface potential but does not change the surface charge density and theelectric field, so that the image is still not developable. Accordingly,in the next step, the surface is charged once again to a constantpotential, using the same procedure as the first step. As a result,additional charges are deposited wherever the exposure had resulted in amigration of charge. The electric field now increases in these regions,producing a spatially varying field pattern and, hence, a developablelatent image. In the fourth step, this latent image is developed byheating the thermoplastic uniformly to a temperature near its softeningpoint. This is done most conveniently by passing a current brieflythrough the conductive coating on the substrate. The thermoplastic layerthen undergoes local deformation as a result of the varying electricfield across it, becoming thinner wherever the field is higher andthicker in the unexposed areas. Once the thermoplastic layer has cooledto room temperature, this thickness variation is frozen in, so that thehologram is quite stable. Because the latent image is relativelyinsensitive to exposure to light after the second charging, it ispossible to monitor the diffraction efficiency of the hologram duringdevelopment and to terminate the application of heat at the proper time.Finally, when the plate is to be re-used, it is flooded with a properwavelength of light, and the thermoplastic layer is heated to atemperature somewhat higher than that used for development. At thistemperature, the thermoplastic is soft enough for surface tension tosmooth out the thickness variations and erase the previously recordedhologram. A blast of compressed air or dry nitrogen is then used to coolthe thermoplastic material rapidly to room temperature, in preparationfor the next exposure. Alternatively, a solvent vapor may be used tosoften the thermoplastic. This has the advantage that it eliminates theneed to heat the substrate. In addition, it gives higher sensitivity andlower noise. Enhanced sensitivity can also be obtained by the use ofdouble-layer and triple-layer photo-conductor systems.

An infrared semi-transparent material, such as a thin gold layer or atransarent electrode, or a suitable organic photoconductor, may bedeposited on the exterior surface of the thermoplastic. Besides servingas an electrode to the photo-conductor it also acts in a manner tofilter the image. The stressed or unstressed states of the infraredimage from the test device is exposed and fixed in the thermoplasticmaterial. Thereafter, if an infrared sensitive camera views thethermoplastic material when exposed to the opposite of the stressed orunstressed state of the infrared image from the test device. The resultis a set of fringes on the thermoplastic material corresponding to themaximum and minimum intensity distribution. Computer aided evaluation ofthe resulting thermoplastic holographs or inferograms may be used, asdesired. If an infrared semi-transparent material is used then theinfrared camera may be used to view the thermoplastic material from thefront side through the transparent photo-conductor, as the infraredsemi-transparent material, such as thin semiconductor material or glass,will make it simpler to detect the fringes.

One approach to making thin-phase thermoplastic holographic recordingsis to sensitize the photoconductor to 1.15 μm infrared light. Thephotoconductor is formed by adding a sensitizer to to polyvinylcabazole(PVK); the dye 2,4,7,-trinitro-9-fluorenone is widely used as asensitizer with visiable light. Since the photoconductor formed withthis dye in complex with PVK is not sensitive to radiate at 1.15 μm,other dyes must be used. The PVK can be sensitized to near infrared byeither of two sensitizers, 2.4,5,7-tetranitrofluorenone or(2,4,7-trinitrofluorenylidene)-malononitrile. A photoconductor solutionis prepared by dissolving PVK in tetrahydrofuran and adding sensitizerat a wieght ratio of one part sensitzer to four to ten parts PVK. Theholographic device is prepared by dip coating the glass substrate andits transparent electrode with the photoconductor. A thermoplastic layeris added by a second dip coating with an ester resin dissolved innaptha. Solution concentrations and dip speeds are adjusted tophotoconductor and thermoplastic layer thicknesses of approximately 2and 0.8 μm, respectively. The sensitizer forms a complex with the PVKthat appears to the eye as dark brown or gray.

Referring to FIGS. 17-19 illustrates what the interference fringes mayappear as. Based on the location and pattern of the fringes, togetherwith a knowledge of the anticipated associated structure of thesemiconductor device, the voltage patterns within the device forparticular structures may be determined. Based on the set of voltagepatterns, the designer may then troubleshoot the construction of thedevice. In this manner, the designer may obtain in a relativelyefficient manner a set of voltages internal to the device and infer fromthose voltages the potential source of any design or fabricationanomalies, as desired.

While it is potentially conceivable to use infrared photographic filmsas a recording medium they are not especially suitable for severalreasons. First, infrared photographic films sense and record theinfrared light as visible light when developed, and not as non-viewableto the human eye infrared light. Typical infrared films record theinfrared light to a maximum of 1,314 nm, where the developed image(Bragg cells) on the film is in visible light (550-800 nm) and hence thevisible image cannot refract or defract infrared light. Second, typicalinfrared photographic films have limited sensitivity and tend to fog dueto chemical sensitivity. The fogging is principally because ofbackground thermal radiation. Third, after its spectral sensitization, aphotographic plate is subject to thermal background radiation during itspreparation and storage, and the various stages in the photographicprocess. This gives rise to a fog and reduces the “lifetime” of a plate,and is the main obstacle to the achievement of reasonable photographicsensitivity. What would be desirable is an infrared “film” type recorderthat senses infrared light and records the infrared light as an infraredimage.

In addition to recording ultraviolet holograms of voltages and plasmasin free-metals with conventual photographic and holographic films, thepresent inventor has identified serveral infrared holographic recordingmediums for recording plasma and voltages in semiconductors using singleand dual wavelength for Gabor-type, in-line holography, off-axisholography, and holographic interferograms. A Bismuth recording film hasdemostrated good senistivity for recording infrared holography forwavelengths from 1.06 μm to 10.6 μm. A film of Magnesium-Bismuth (MnBi)has demonstrated good recording characteristics at 1.06 μm. Plexiglashas been used to record infrared holograms at 10.6 μm.

The present inventor came to the realization that a controlling factorshould be included with the recording medium to permit the recording ofinfrared light only while the medium is “turned on,” so that therecording material is sensitive only when a useful image is projected onit. In addition, at the end of the exposure, the factor should be“switched off” so that the material remains insensitive throughout thesubsequent storage of the exposed material. Thus the basic principle ofcontrollable sensitivity is the sensitization of a photographic materialonly during its exposure. Also, if desirable, the controlled sensitivitymay be switched on and off only in a certain spectral region (controlledspectral sensitization). The present inventor further came to therealization that semiconductor materials exhibiting a photoelectricsensitivity in the infrared range may be used as a recording device. Itneeds to be understood that in electro-photograph recording techniquescharge dispursion and diffusion are limiting factors on imageresolution. The use of voltage channeling conductive elements and theirsubsequent arrangements are to be applied to the recording devicesdescribed herein, as desired.

One potential semiconductor material may include bringing into contact,during the exposure time, two separate parts of a photographic system,each of which is not photographically sensitive on its own but only whenin contact with the other material. For example, a thin photosensitivesemiconducting film may be brought into contact with an aqueouselectrolytic solution. However, the electrolytic solution and thephotosensitive semiconducting film tends to be, by itself, insensitiveto infrared light. When the two materials are brought into contact, anoxidation-recombination reaction takes place at thesemiconductor-electrolyte interface. The rate of the reaction isdependant, at least in part, on the electron (hole) density in thesemiconductor, i.e., on the intensity of illumination falling on thesemiconducting film. The reaction may be ended by breaking the contactof the two pieces, or any other suitable technique. Thus, thesemiconductor film is again insensitive to infrared radiation.Alternatively, a semitransparent film of lead sulfide evaporated on aglass substrate could also be used as a photographic plate. Combinedwith a germanium filter, this plate may be exposed to obtain an opticalimage, or the exposure stopped in the latent image stage and the opticaldensity could then be increased by “physical” development.Alternatively, a thin semiconductor plate of n-type gallium arsenide incontact with an electrolyte solution containing HNO₃ produces an imageby selective photo etching of the semiconductor surface. However, thisrequires a physical manipulation of the parts, which is not highlydesirable.

An alternative structure includes electrically controlled processeswhere an “electric shutter” is used to “switch on” the photosensitivityduring exposure. There are two general techniques to electricallycontrol the photographic process with a self-contained current-sensitivefilm, namely, a liquid process and a process with a self-containedcurrent-sensitive film. Referring to FIG. 20, a semiconducting film 602,covered by a conducting transparent layer 600, (such as for exampleglass, Mylar, or a semiconductor film) is brought into contact with anelectrolyte solution 604 bounded on the opposite side by a metal counterelectrode 606. An optical image is projected on the outer surface of thefilm 602. Then, an electric voltage from a power supply 608 is appliedto the electrodes 600 and 606 by closing a switch 610. A latentphotographic image is formed because of the difference in the rates ofelectrolytic deposition of a metal from the electrolyte solution on theilluminated and unilluminated parts of the semiconducting film. Thisimage can then be intensified in a “physical” developer. Many materialsmay be used for the semiconducting film 602, such as for example,silicon and germanium plates as well as lead sulfide films deposited ontransparent conducing layers of SnO₂.

Another further embodiment of the image is not formed on the surface ofa semiconductor but in a self-contained current-sensitive film.Referring to FIG. 21, a semiconducting film 622 has a protective coating624. This coating is a composite material based on an epoxy resin withconducting inclusion where the conductivity of this coating isanisotropic. The anisotropic coating is pressed tightly against acurrent-sensitive film 626, consisting of a gelatine layer on aliquid-permeable base impregnated with an electrolyte solution. Theopposite side of the current-sensitive film is covered by a counterelectrode 628 in the form of a metal foil. The photosensitivesemiconducting film is a large-area surface-barrier p-n, n-p-n, orp-n-p, junction with a high-resistivity bulk region. The illuminatedside of the semiconducting film has an ohmic contact 620. When anexternal voltage is applied to the film (such as for example glass,Mylar, Kodar, etc.), the junction is biased in the reverse direction.The junction, whose load is the electrolytic cell 624, 628, operatesunder the photo-diode. The image is formed by electrolysis in thecurrent-sensitive film 626. The principal advantage of this system isthe repeated use of the photosensitive element. The system may be usedwithout an external voltage source if the electrolyte composition andthe electrode materials are chosen in a suitable manner. In this case,simple closing 632 of the circuit's power supply 630 provides the“electric shutter” action. The developed film can be subsequentlyapplied or fixed to an optical surface or device.

Yet another alternative embodiment of a semiconductor as the basis ofthe film consists of the following. A charge is uniformly distributedacross the surface of a high-resistivity semiconductor placed on aconducting substrate. The charge leaks away from the illuminated regionsbecause of their photoconductivity and a latent electrophotographicimage is formed on the plate. The image is made visible by developmentinvolving the precipitation of the charged colored particles of thedeveloper on the unilluminated parts of the image. The photographicsensitivity of the plate is absent during its preparation and isimparted by charging the plate. The charging process acts as acontrolling factor which “switch on” the sensitivity. However, thelatent image has only a limited lifetime and the sensitivity can not be“switched off” after the exposure. The process can be completed byelectrostatic development.

There are additional techniques that may be employed. The effect on aphotographic plate of the background thermal radiation during theplate's storage and preparation before its exposure and during itssubsequent treatment can be suppressed by exploiting the uniformilluminance of this radiation. A process can be developed in which thephotographic effect is obtained only by projection of an image withregions of different illuminance but not by uniform illumination. Aphotographic process of this type may ensure insensitivity of aphotographic material to the fogging effect of the thermal radiation andstill give an optical image. One of possible variants of such a processinvolves the use of the photo emf in semiconductors, p-n, n-p-n, orp-n-p, junctions for the purpose of obtaining a photographic image.Referring to FIG. 22, two photo-diodes whose p-type regions areconnected by a metallic conductor and whose n-type regions are connectedby an electrolyte EL. When the illuminance I1 of both junctions is thesame, they generate identical photo emf's and there is no current in thecircuit. However, if an additional light flux I2 reaches one of thesephoto diodes, this diode produces a photo emf V2 and an electric currentflows in the circuit. The electrode of the more strongly illuminatedphoto diode acts as the cathode. Neutralization of the metal ions in theelectrolyte, which then form an image, occurs near the cathode.

Other suitable materials for this process include a base layer ofn-material Aluminum overlaid by layer of p-material silicon which woulda photovoltaic response from ultraviolet to approximately an infraredwavelength of 1.1 μm. Since the Aluminum layer is reflective to infraredradiation, it is useful in making films for infrared reflection(non-transmission) holograms. Both layers can be plasma deposited onto ahigh refractive index wafer or quartz (glass) surface (with the Aluminumlayer outward) so that the hologram is recording through the quartz. Theimage quality of the developed hologram can be enhanced by the addition,a thin (or at wavelength specific thichness) dark infrared lightabsorbing layer of either p-, n-semiconductor or dielectric materialscan be sandwiched between the silicon and aluminum layers. Ininfrared-transparent semiconductor materials, the materials' higherindex of refraction gives a higher line resolution per millimeter in theprocessed film.

It is possible to increase the resolution of the aforementionedsemiconductor based recording devices by the combination of an infraredwave front (or a relatively equal ultraviolet wave front strength to theinfrared wave front) and e-beam convergence on the semiconductor inplace of an applied voltage.

A further embodiment includes a recording device including apolycrystalline film, and in particular polycrystalline film includinglead sulfide and selenide.

Another embodiment includes a lead sulfide recording device includingsensitivity-controlling by an electric field applied to asemiconductor-electrolyte system where the image is formed directly onthe semiconductor surface. A lead sulfide film may be prepared by vacuumevaporation followed by annealing. For example, the films may bedeposited on a glass substrate with semitransparent tin dioxideelectrodes (such as a surface resistance below 50 ohms/square). Duringexposure the semiconductor film is brought into contact with anelectrolyte. The exposure is made through a film, such as a germaniumfilter. The light is focused onto the surface of the PbS film. For bestresults the electrolyte should have the following characteristics: (1)the effect of light should not result in the formation of an image inthe absence of an electric field; (2) the selectivity of the electrolyteshould be high in contrast to the electrolytes employed inelectroplating; (3) the electrolyte should have a high currentefficiency and the electrolysis should give rise to heterogeneouscatalysis centers necessary for efficient development, (4) theelectrolyte should produce an image with a high color contrast on thegray background of the semiconductor film. One potential electrolyte isa solution of simple salts, such as lead or silver nitrates, coppersulfate, or calcium chloride. A potential explanation of the mechanismof formation includes consideration of the semiconductor-electrolyteinterface in an electric field. Cathodic polarization of thesemiconductor surface produces a negative space-charge region and acorresponding blocking barrier at the surface of the p-typesemiconductor. The selection of the elements of the light sensitivePbS-electrolyte system consists of addition of oxidizing or reducingagents to the electrolyte in accordance with the type of conduction ofPbS. Referring to FIG. 23, during exposure to light the voltage appliedin the blocking direction alters the sign of the surface potential. Thephoto-excited electron-hole pairs are split by the space-charge field.The electrons neutralize the positive electrolyte ions and form ametal-deposit image on the semiconductor surface. FIG. 23 illustratesthe energy band scheme of a semiconductor to illustrate the appearanceof an anti-blocking barrier at the boundary with an electrolyte, wherethe left side is in the absence of an electric field and the right sideis during application of an electric field in the blocking direction.

Yet another alternative infrared recording medium is aphoto-conduct-o-graphic system. The principal advantage of such a systemis the formation of an image in a separable film and the consequentreusability of the photosensitive layer. One potential material utilizeshigh-resistivity gallium arsenide without a protective coating which isa compromise between photo-conduct-o-graphic andsemiconductor-electrolyte systems, combining the advantages of both.Referring to FIG. 24, an image is exposed on a photo-conductor 680 onthe side covered by a semitransparent electrode 682, preferably made ofnickel. A film 683, such as cellophane, Mylar, or Kodar, is imprinted(or otherwise supports or includes) an electrolyte so that it issuitable for carrying current is pressed to the opposite side of thephoto-conductor, preferably with imprinted voltage dispursion anddiffusion restricting structures. The area of this film is greater thanthat of the photo-conductor and the projecting part of the film is usedto make contact with a conductive counter-electrode 684, such as acopper or graphite electrode. During exposure the semi-transparentelectrode is subjected to a negative potential and the counter-electrodeto a positive potential from a power source, such as a battery 685.

The counter-electrode (anode) is located at such a distance from thephoto-conductor that the products of the anodic reaction in thecurrent-carrying film near the counter-electrode can not diffuse intothe main part of the assembly and spoil the useful image by accidentalblackening.

The use of high-resistivity gallium arsenide eases the requirements thatthe electrolyte must satisfy in respect of the differential resistancein cathodic reactions, i.e., the resistance of the current-carrying filmcan be relatively high. When a high-resistivity photo-conductor is usedin direct contact with a current-carrying film (without a protectivelayer) it is found that the system described above operates more or lessefficiently with electrolytes. It follows that the protective layer neednot be used, if desired.

The principal photographic characteristics of the system are a functionof the electrolyte used and particularly of the ability of thelatent-image centers to become localized in the current-carrying filmduring exposure and to remain in this film for some time after theexposure. These characteristics depend also on the process responsiblefor the visualization of the nonmetallic latent-image centers. Forexample, if a Phenidone electrolyte is used, the latent-image centerslocalized in the film may remain for several minutes and during thistime the latent image is not affected by ordinary or acidified water.When the electrolyte is a weak aqueous solution of a neutral salt, thelatent-image centers localized in the film have a sufficient degradationtime. This time is sufficient for retention of the latent image from theend of the exposure to the beginning of the visualization process.

There may be at least two general types of reactions between asemiconductor material and molecules of an adsorbed reactant, controlledby active radiation. First, the chemical reaction between the componentsof the reagent is catalyzed by non-equilibrium carriers from thesemiconductor. The visual image is produced by heterogeneous-catalysisreaction products. Second, the reaction or oxidation of the reagent onthe semiconductor surface, involving non-equilibrium electrons or holes,respectively, and the chemical reaction of the products with thesemiconductor material. The image is produced either through localetching of the surface, when the reaction products are soluble orgaseous and are removed from the surface, or by colored insolublereaction products adsorbed on the surface. For example, a gasphoto-corrosion of thin semiconducting films of indium antimonide may beused. Since the bonds in A^(III)B^(V) compounds are covalent, a strongoxidizing agent such as nitric acid (vapor) may be used as a constituentof the etchant. The deep penetration of the corrosion reaction, to adepth where the distribution of lattice defects is still significant,may reduce considerably the absorption of visible light in a thin film,if the reaction products are removed from its surface. Alternatively, animage may be formed by using a colored film of insoluble oxide on InSb.The system my include indium antimonide and nitric acid vapor togetherwith polycrystalline InSb films 0.5-1.5 microns thick, deposited byevaporation on glass substrates. The following model may explain theprocess. Initially, the contact between the InSb and the nitric acidvapor produces a poly-molecular adsorbed layer of nitric acid. Thereduction of the nitric acid by equilibrium electrons (minoritycarriers) is slow. Optical excitation causes a sharp increase in theelectron density at the surface, and the reaction becomes much faster.The adsorption equilibrium between the InSb surface and the nitric acidvapor is shifted in the direction of further adsorption of the reagentmolecules. The equation may be as follows 3H⁺+NO₃ ⁻+2e⁻--->HNO₂+H₂O. Theproducts of the reaction interact with indium antimonide and form, atpoints where there is location illumination of the semiconductor, ablack film of oxide, which constitutes the image. This photographiceffect may also be observed in photo-sensitive films of lead sulfideexposed to a mixture of hydrochloric and acetic acid vapors.

An extension of the aforementioned semiconductor based photography intoinfrared wavelengths may be achieved if the photo-sensitive element iscooled. For example, consider a film of semi-insulating GaAs is dopedwith zinc, 100 microns or less thick, and its resistivity generally 10⁸ohms cm. The dielectric layer may be a film of polyethylene 10 micronsor less thick covered by a conducting coating. The semiconductor and thedielectric layer are pressed together between conducting electrodes andimmersed in liquid nitrogen to achieve cooling, if desired. Preferably,the liquid nitrogen is evaporated from the gap between the semiconductorand the dielectric. Then simultaneously the system is illuminated andsubjected to a voltage pulse. The charge is transported across the gapbetween the dielectric and the semiconductor, which was filled withgaseous nitrogen. The dielectric may be extracted from the system anddeveloped in a developer which can be subsequently fixed to an opticalsurface or device. In addition, other thin dielectric mediums may beused, such as doped Mylar or Kodar films.

Referring to FIG. 25, yet another alternative recording medium includesa semiconductor photo-detector film 691, which has a transparentconducting contact 692 and a protective layer 693 (on the outer surface)whose conductivity is preferably anisotropic. The protective layer isbrought into intimate contact with a current-sensitive electrochemicalfilm 694, which has a counter-electrode 695. An image is projected onthe semi-conductor film 691. When the electric circuit is closed by aswitch 697, a latent image is formed by electrolysis in thecurrent-sensitive film 694 because of the differences between thecurrent density in the illuminated and unilluminated parts of thesemiconductor film 691. Preferably, the photo-sensitive element is asurface-barrier p-n, n-p-n, p-n-p, junction made of p-type or n-typesilicon. The p-n, n-p-n, or p-n-p junction used in such a photographicsystem should have a high resistance and a small area in contact withthe anisotropic layer. This assists in preventing appreciable spreadingof the current, which would affect the resolving power of the system.Account is taken of the influence of various treatments on the surfaceband bending in the protective layer. The required characteristics areachieved by etching the silicon surface and heat treating the protectivelayer, which is a compound based on an epoxy resin containing a fillerand a conducting component. The current-sensitive film should be easilyremovable from the protective layer and capable of further developmentwith a minimum of fogging. Moreover, the current-sensitive film shouldbe sufficiently rigid in order to avoid any distortion of the imageduring subsequent treatment. These requirements are reasonably wellsatisfied by a film of tanned gelatine with an admixture of glycerine ona base which is permeable to liquids. After the immersion of such a filmin an electrolyte solution and subsequent brief drying, its consistencyshould be such as to ensure a satisfactory contact with, and removalfrom, the surface of the anisotropic layer.

Yet another recording material including semiconductor materials withforbidden bands narrower than the forbidden band of silicon. Inaddition, this may be extended to polycrystalline films, if desired. Forexample, a reaction of lead sulfide or selenide with an aqueous solutionof AgNO₃ may be used. The rate of the reaction is based, at least inpart, for p-type PbS or PbSe by the rate of the cathodic component,i.e., by the rate of precipitation of silver. This precipitation rate isdetermined by the density of non-equilibrium carriers (electrons) in thesurface layer of the semiconductor; consequently, it depended on theillumination. The precipitation of silver was thus localized on theilluminated parts of the sample, whereas the sulfide or selenide wasdissolved in the unilluminated region. In the case of silicon, the imagemay be projected on that surface of the sample which is opposite to thesurface in contact with the electrolyte. Pbs films may be evaporated invacuum on glass substrates and activated by heating in air. Thethickness of the films is preferably such that they are semi-transparentafter activation. This makes it feasible to obtain the image bytransmission.

A two part system shown in FIG. 26, based upon a p-n silicon andaluminum junction with photoelectric IR senistivity to 1.1 μm, it ispossible to utilize an IR reflective backplane material 900 (Aluminum)and a top layer 902 of silicon. The device, and all other recordingmaterials, can be mounted on 904, a transparent material such as asemiconconductor or a glass substrate having a high index of refraction.FIG. 26 showns a three-part system based upon silicon-aluminum p-n-p orn-p-n junction. In this device, one layer is doped in two differentconcentrations of p- or n-material. The top layer 906 being IRtransmissive silicon, the middle 908 layer of either silicon or aluminummaterial is doped as to contrast to the 906 silicon layer and the bottom910 Aluminum layer so that a p-n-p or n-p-n junction is created having912/914/916 electrical contacts to control each layer in the three-partjunction.

The inventor then realized that these two-part or three-part systemfilms satisfies the requirements for making thin, phase, thin-amplitude,thin-phase, and thin-phase-reflection holograms, the films did notreadily meet the requirements for creating volume- andvolume-transmission, and volume reflection holograms. The inventor thenrealized that IR volume holograms could be readily achieved if an IRrecording material could created which was able to record and resolvegreater amplitude and refractive-index values.

Refering to FIGS. 26a and 26 b p- or n-doping dark (or light absorptivematerial and over lay it with a infrared transparent material suchsilicon or 3/5 materials (2/7 materials are possible also).

In FIGS. 27a and 27 b show an approach to enhancing the probing beam'ssensitivety in FIG. 1 by the application of IR holographicinterferometric gratings to form the incoming beam or plane waveincident to the LiTa and IR holographic gratings to the object beam fromthe LiTa probe.

The present inventor came to the realization that temporary transientvolume holograms utilizing a spatially modulated free carrier patternwould be useful in providing real-time hologram recording andinterferometric evaluation of TO 224. When a photoconductor receivesnonuniform illumination. e.g., illumination limited to just a smallportion of the interelectrode spacing, some special effects can beobtained. In general, insulators subjected to such non-uniformillumination would be expected to show a negible fractional increase inphotocurrent because the flow of the current would be effectivelyprohibited by the buildup of space charge. The same is true ofsemiconductors, if carriers of only one type are mobile, but not if thecarriers of both types are mobile.

Shown in FIG. 28 is a transmission holographic setup for recordingvolume holograms of TO employs temporal free-carriers recorded within asolid state recording medium.

The effects which can be obtained with a nonuniformly excited insulator.As indicated in FIG. 29, consider a photoconductor illuminated parallelto the applied electric field, e.g., through a partially transparentelectrode, by light which strongly absorbed and produces excitation onto a depth d. The excitation of free carriers in this portion as uniformover the distance d; the excitation of the free carriers in this portionof the crystal creates an effective barrier which will have the formshown in FIG. 30 the electrons are the majority carriers. Ifreplenishment of electrons from the dope semiconductor p- or n-layeredmaterials is not possible, then a small space-charge-limited electroncurrent will be drawn when the illuminated electrode is negative, and aneven smaller current when the illuminated electrode is positive, becauseof the much smaller assumed effective mobility of the holes. But, ifreplenishment of electrons at the cathode is possible, the thespace-charge-limited current which flows for the illuminated electrodenegative may well be smaller then the current that flows for reversepolarity for the following reason: When the illuminated electrode ispositive, photoexcited holes may diffuse under action of the field intothe unilluminated portion and there be trapped to provide a positivespace charge; electrons enter the crystal from the photocurrent,continues until recombination occurs between a free electron and thehole. It should be noted that the field direction for the maximumcurrent flow for the same material with the same same sigh of majoritycarriors can be reversed, depending on whether or not electrons can bereplenished. FIG. 29 shows a schematic and energy-level diagram of anonuniformly excited photoconductor

A similar set up for the type of photocurrent (primary or secondary) tobe expected from nonuniform illumination of a photoconductor. Thisdiscussion leads naturally to the reason why nonnegible photocurrentscan occur in semiconductors with both carriers mobile, even fornonuniform illumination. Shown in FIG. 31 is a schematic representationof a photoconducting crystal excited nonuniformly. The photoconductingcrystal with neutral contacts in which

(1) only electron current need be considered, the holes being rapidlytrapped at or near the site of their creation, and

(2) electron-hole pairs are generated at a distance x from the anode bya pulse of excitation.

The trapped holes from a space-charge of +Q cm{circumflex over ( )}−2,and their presence causes electrons to enter the crystal from thecathode. If the excess electrons which this enter the crystal constitutea charge of −aQ cm{circumflex over ( )}−2, the effect of the net chargeis to increase the field at the cathode$E = {E_{o} + {\left( \frac{x}{d} \right) \cdot \left( \frac{4\pi}{e} \right) \cdot Q \cdot \left( {1 - \alpha} \right)}}$

where electrostatic units have been used, Eo is the field in the absenceexcitiation, and e is the diaelectric constant. The increase in currentdensity corresponding to the increase in fild is given by${\Delta \quad j} = {\mu \quad {n \cdot {e\left( \frac{x}{d} \right)} \cdot \left( \frac{4\pi}{e} \right) \cdot Q \cdot \left( {1 - \alpha} \right)}}$

This increase in current may also expressed as${\Delta \quad j} = {\mu \cdot {E_{o}\left( \frac{\alpha \quad Q}{d} \right)}}$

Equating these two relationships for Dj shows that$\alpha = \frac{\beta}{1 + \beta}$

where $\beta = \frac{4\pi \times {en}}{{eE}_{o}}$

If strongly absorbed light or radiation is used to excite the material,x˜d, and then examination of the pevivous expression β shows thatquanity b can be expressed as the ratio of two times

(1) the transit time, and: $\tau_{n} = \frac{d}{E_{o} \cdot \mu}$

(2) the the dielectric relaxiation time [81] (essentially the RC timeconstant) of the material:$\tau_{r} = \frac{10^{- 12} \cdot \rho \cdot e}{4\pi}$

where we have inserted the numerical factor to make the expressionusable with conventional units for the resistivity r and e. Thus$\beta = \frac{\tau_{n}}{\tau_{r}}$

so that the ratio of secondary to primary photocurrent is$\frac{\Delta \quad j}{j_{p}} = \frac{\beta}{1 + \beta}$

To measure a primary photocurrent without any contribution from thesecondary photocurent, then β must be much less than unity; i.e.,tr>>tn. All other quanties being about the same, this means that a pureprimary photocurrent can be observed in material only with aresistivity, i.e., insulators.

The same kind of reasoning is involved in determining the typephotoconductivity which results from nonuniform illumination of amaterial in which both carriers are mobiles. The key question is this:When electron-hole pairs are generated in a material by nonuniformillumination, do the electrons and holes separate, each going to theappropriate electrode, or do the photoeexcited minority carriers movetoward their approapriate electrode and taske along with them an equalnumber of majority carriers to maintain an absence of space charge. Theanswer is that, tr>>t_(maj), the carriers move in oppposite directions;this is the condition found in insulators where nonuniform illuminationproducts a space-charge limitation on the current. In homogenousphotoconductor materials, when both carriers are mobile but the holesare not replenished at the anode, the gain is given by${Gain} = \frac{\mu_{n} + \mu_{p}}{\mu_{p}}$

If t_(maj)>>t_(r), on the other hand, the minority carriers take withthem an equal number of majority carriers, and and the photocurrentresults from this slice of increased conductivity; this is the conditionfound in semiconductors where nonuniform illumination is not to verysmall values by space-chare limitiations if both carriers are mobile.The actual gain of the photocurrent is the same as the previvous case[for the definition of Gain]. In the later case, the the space-chargewhich would be built up by the separation of the charges is rapidilydissipted by normal conduction processes, the neutralization of thecharge being carriered out through those carriers which contribute themost to conductivity, i.e., the majority carriers.

In holographic filtering descrete voltage levels can be targeted,allowing for voltage induced jittering or timing; giving optionaloptical filtering conditions, i.e., negitive (low) background voltagelevel permites target photocurents, a large voltage exceeds the targetedvoltage level recorded by the holographic filter.

Shown in FIG. 33, is an Energy level diagram showing external voltageclocking or shuttering a photoconducting crystal and the targetedphotogenerated voltage levels excited nonuniformly. The invasive orphotogenerating nonuniform light is correspondingly clocked. Thephotoelectrically generated pattern is read using noninvasive(non-photogenerating) light. Photoelectric materials and behaviors;polarized light effects, photoelectric noise sources, drift mobilities,negitive photogeneration (useful for reversed-engineered or negitivephotoactive surfaces), photodielectric effect (useful formacro-elctronic capacitor devices or in display, telemetric devices, aswell as in both analog-to-digital and digital-to-analog converterdevices which require longer lived voltage waveforms), and measurementof lifetime and diffusion length.].

The free-carrier pattern acting as a pure-phase volume hologram anddecays by diffusion of the free carriers using an apparatus as shown inFIGS. 5a/5 b/ 7/8/14 a/ 14 b/ 14 c/ 26 a/ 26 b/ 27 a/ 27 b/ 29/30/31/.When a beam from a Q-switched Nd:YAG laser (λ=1.06 [goto 1.3 μm],hv=1.16 eV [need to lower this photogenerating threshold by selectivelydoping semiconductor materials]) passes through a thin slice [subsistuteSi with a semiconductor such as Cd with lower E₈ values] of Si(E₈=1.11eV), a large concentration of free electrons and holes are created byoptical interband transitions [layer the Cd material with one other ormore p- and n-doped semiconductors to increase interband transistions](Lowering the Applied laser power levels from ≈3×10¹⁷ cm⁻³ for a powerflux>1 mJ/cm⁻² and a pulse length of 10⁻⁸ sec by selective substratedoping of p- and n-materials will significantly reduce the possiblityfor laser damage to the TO). Temporary transient volume hologramrecording parameters: (1) thickness of the silicon wafer slice, (2)energy flux (mJ/cm²) from laser incident upon the hologram plane, (3)the angle θ between object and reference beams, and (4), the ratio ofthe optical set-up's propergation delay and the duration of the laser'sinitial optical pulse. Additional optical resolution is available byemployiong a ½ waveplate in front of the mirror so that incident andreflecting waves do not destructively cancel each other resulting inconstructive free carrier field gains. All of the disclosed holographicand photographic films and recording devices can be utilized to enablefree carrrier, non-chemical development, examination of TO 224.

Two-wavelength holographic image production utilizes the interaction ofphotovoltages to produce computer generated holograms into voltagepatterns on the free-carrier optical element to create a free-carrierimage by a visible or UV wavelength which is subsequently read byanother wavelength of non-photoelectric generating IR light. Serveralsuitable techniques are discribed later is in this disclosure for makingcomputer generated holograms of devices and conditions to beholographically tested. The amplitude and phase patterns of the shortphotogenerating wavelength are optically enlarged so that theycorrespond directly to the wave front dimensions of the IR opticallyread hologram. The photogenerating wavelength incident to free-carrierrecording medium can be created or generated using, but not limited to,display devices such as cathode, liquid crystals, gas-plasma, or otherfree-carrier based devices.

Holographic filters in conjunction with holographic (initialpolarization recording) mapping of the active optical surface are usedto observe by non-invasive polarized light—the facsimile voltagepatterns correspond to the optical amplitudes and phases that create theoptical wavefront of the holographic image to be displayed. An electronlens or a cathode-ray element can be used to electrically reduce thevoltage patterns down and transfer them to another optically activesurface to increase image resolution if necessary.

Another approach is to use the following holographic technique offour-wave mixing utilizing free carriers and optical frequency doublingto enable real-time phase-conjunctent (4-wave mixing) examination of TO224 shown in FIG. 35.

In addition, other films may be used, as desired.

Additional recording mediums also include Bi₁₂SiO₂₀ (BSO) or Bi₁₂GeO₂₀(BGO) electro-optic recording devices sensitized to infrared byenclosing the BGO or BSO crystal in a vacuum chamber (having infraredtransparent windows) which eliminates air induced resistances toinfrared generated photo-currents. These electro-optic devices canenable four-wave mixing or phase conjugate imaging holographictechniques to be applied to the inspection and voltage characterizationof TO 224.

IR volume holograms offer to semiconductor voltage test andcharacterization, the ability to record two different transparencies(each formed by a different recording wavelength) on the same recordingmedium for subsequent reconstruction without crosstalk. The IR volumehologram recording process can be varied to also enable same-wavelengthmultiple hologram storage by rotating the recording medium after eachexposure. Another suitable technique of essentially multiplexingmultiple-images on a single recording medium is by considering thecontext of color holography; different non-overlapping regions of thesame photographic plate can be utilized to record IC (TO 224) hologramsof different substrate voltage signals, recording beam angles,wavelengths and beam polarizations. Unlike conventional holographic beamrecording techniques where the a incident beam of identical polarizationis required to reconstruct the hologram, in polarization holographyhologram reconstruction requires both the recording wavelength and beampolarization.

Pattern Recognition

Holograms created by computer calculations provide opportunity togenerate wavefronts of any prescribed amplitude and phase distribution;this has been demonstrated to be extremely useful for generatingthree-dimensional images, testing optical surfaces, opticalspatial-filtering, laser scanning, as well as pattern recognition ofsemiconductor devices and substrate voltage waveforms. Production ofthese holograms employs a discrete Fourier transform to generate thecomplex amplitude and phase distributions of an array of N_(n)×N_(n)elements. Each element of the image is broken into Fourier coefficientswhich are computed using the fast Fourier transform (FFT). Inholography, the second step follows which produces a transparency (thehologram) which reconstructs the object wave when suitably illuminated.Any dynamic changes in the test object's transparency away from ideal orrecorded conditions, such as by defects in the TO's circuitry or fromprescribed voltage operating parameters, will not result in thereconstruction of the desired object wave or holographic image. Refer toFIG. 36.

Digital Processing

The Fourier transform describes functions into different dimensions orcoordinates such as Cartesian to spherical. For example, a functioncould be represented in the domains of time and frequency. The conceptof the 3-D FFT has the same form as the mathematical representation ofthe 2-D FFT utilizing a 1-D FFT. Here, an element (n1, n2, n3) of the3-D array (N1, N2, N3) would be defined in a 1-D device where:

n=N ₁ ·N ₂ ·n ₁ +N ₃ ·n ₂ +n ₃   (87)

Similarly, the 3-D FFT can derive from a time sample x(k1, k2, k3) afrequency sample X(k1, k2, k3). The parameter N can be expressed as:

N=N ₁ ·N ₂ ·N ₃   (88)

Character & Condition Recognition

This property of associative storage has been used for recognizing acharacter with a hologram, it is applied to recognize the presence of aspecific electronic circuit and detect specific voltage levels withinthe TO circuit. It makes it also possible to use an isochromatichologram to perform a recognition of voltage condition(s) present intemporal voltage waveforms and in selected microelectronic devices andcircuits. Like the previous section, this technique is essentially aspatial filtering operation in which the hologram functions as a matchedfilter. The approach here is to (1) couple invasive beam(s) to anelectrooptic modulator to a prism, (2) invasively write the spectrallyresolved beam(s) to the optically active surface to spatially filter,(3) read this with a noninvasive beam, (4) correlate the spatiallyresolved beam with the hologram (either a fixed hologram or a temporalphotogenerated voltage field) with this noninvasive beam which istransmitted to invasively write the temporal photoelectric-pattern'scorrelation with the hologram on another optically active surface.

A optical system for this is shown in FIG. 36. To produce the matchedfilter, a transparency of TO's targeted circuit components at thedesired voltage levels to be identified is placed in the input plane anda TO hologram of this IR transparency is recorded in the Fouriertransform plane using a point reference source. For simplicity, weassume that the complex amplitude of the input place due to the TOtransparency is a one-dimensional distribution $\begin{matrix}{{f(y)} = {\sum\limits_{j = 1}^{N}\quad {f\left( {y - c_{j}} \right)}_{j}}} & (82)\end{matrix}$

where f(y−cj)j is the complex amplitude due to a typical charactercentered at cj, while that due to the reference source is δ(y+b).

If we assume linear recording, the transmittance of the hologram can bewritten as $\begin{matrix}{{t(\eta)} = {t_{o} + {\beta \quad \cdot T \cdot \begin{bmatrix}{\left\lbrack {1 + \left( {{F(\eta)}} \right)^{2}} \right\rbrack \quad \ldots} \\{+ \begin{pmatrix}{{{F(\eta)}_{\alpha} \cdot {\exp \left( {{- } \cdot 2 \cdot \pi \cdot \eta \cdot b} \right)}}\quad \ldots} \\{{+ {F(\eta)}} \cdot {\exp \left( { \cdot 2 \cdot \pi \cdot {\eta b}} \right)}}\end{pmatrix}}\end{bmatrix}}}} & (83)\end{matrix}$

where

F(η)⇄f(η)   (84)

The hologram is replaced, after processing, in exactly the same positionin which it was recorded and illuminated by single character of the setcentered on the axis. A computer generated hologram (CGH) of TO can becreated utilizing IC layout tools and experimental data as well astheoretical test conditions can be subsisted in its place. If theamplitude due to the target device and its operating characteristics inthe input plane is f(y)1, the transmittance of the hologram is$\begin{matrix}{{{H(\eta)}\quad:={{F(\eta)}_{j} \cdot {t(\eta)}}}{{hence},}} & (85) \\\begin{matrix}{{H(\eta)}\quad:={{{\left( {t_{o} + {\beta \cdot \overset{\_}{T}}} \right) \cdot {F(\eta)}_{1}}\quad \ldots} +}} \\{{{{\beta \cdot T \cdot {F(\eta)}_{1} \cdot \left( {{F(\eta)}} \right)^{2}}\quad \ldots} +}} \\{{{{\beta \cdot T \cdot {F(\eta)}_{1} \cdot {F(\eta)}_{\alpha} \cdot {\exp \left( {{- } \cdot 2 \cdot \pi \cdot \eta \cdot b} \right)}}\quad \ldots} +}} \\{{\beta \cdot T \cdot {F(\eta)}_{1} \cdot {F(\eta)} \cdot {\exp \left( { \cdot 2 \cdot \pi \cdot \eta \cdot b} \right)}}}\end{matrix} & (86)\end{matrix}$

Shown is FIG. 36 is an optical system used for experiments in electronicdevice and voltage condition recognition. In this setup is configured asin FIG. 1 with the embodiment of the lenses and hologram in a singleholographic optical element. Input by the TO is by either directelectron-shading of temporal voltage waveforms or voltage displacementsin epitaxial circuits.

The complex amplitude in the input output plane is then the Fouriertransform of (86), which is $\begin{matrix}\begin{matrix}{{h(y)}\quad:={{{\left( {t_{o} + {\beta \cdot \overset{\_}{T}}} \right) \cdot {f(y)}_{1}}\quad \ldots} +}} \\{{{{\beta \cdot T \cdot {F(y)}_{1} \cdot {\alpha \left( {{{f(y)} \cdot \Delta}\quad {f(y)}} \right)}}\quad \ldots} +}} \\{{{{\beta \cdot T \cdot {F(y)}_{1} \cdot \Delta}\quad {{f(y)} \cdot {{\alpha\delta}\left( {y + b} \right)}}\quad \ldots} +}} \\{{{\beta \cdot T \cdot {F(y)}_{1} \cdot \alpha}\quad {{f(y)} \cdot {{\alpha\delta}\left( {y - b} \right)}}}}\end{matrix} & (87)\end{matrix}$

the only term of interest in (87) is the last but one on the right handside which corresponds to the correlation of f(y)1 with all thecharacters of the set. If we ignore the constant factor βT, this can beexpanded as $\begin{matrix}{{{{{f(y)}_{1} \cdot \Delta}\quad {{f(y)} \cdot {{\alpha\delta}\left( {y + b} \right)}}} = \Phi}{hence}} & (88) \\{{\Phi = \left( {{{f(y)}_{1} \cdot}{\left( {\sum\limits_{j = 1}^{N}\quad {f\left( {y - c_{j}} \right)}_{j}} \right) \cdot {{\alpha\delta}\left( {y + b} \right)}}} \right)}{and}} & (89) \\{\Phi = {{{\left( {{{f(y)}_{1} \cdot \Delta}\quad {f(y)}_{1}} \right) \cdot {{\alpha\delta}\left( {y + c_{j} + b} \right)}}\quad \ldots} + \left( {{f(y)}_{1} \cdot {\Delta \left( {\sum\limits_{j = 1}^{N}\quad {f\left( {y - c_{j}} \right)}} \right)} \cdot {{\alpha\delta}\left( {y + b} \right)}} \right)}} & (90)\end{matrix}$

If the auto correlation function of the character presented is sharplypeaked, the first term on the right hand side of (90) represents abright spot of light, which is the reconstructed image of the referencesource, located at

y=c ₁ −b   (91)

The presence of this bright spot in the output plane corresponds torecognition of the targeted microeleclectronic circuitry andaccompanying voltage conditions present as one belonging to the originalset. The fact that this image is reconstructed at a distance −c1 fromits correct identifies the character presented as f(y−c1)1.

This basic circuit test condition recognition technique has beenextended to permit simultaneous identification of all the circuit testtargets and conditions on a single recording medium to allow multipledevice and condition tests to be carried out in parallel. Shown in FIG.37a and 37 b are the system components.

When real-time operation is not required, a more direct technique can beused. This involves the use two transparencies in the input plane. Oneof these f(y+b)1 is a transparency of the character to be located, whilethe other f(y+b)2 is a transparency of the recording medium of thecircuits and voltage conditions to be searched. The transmittance of theFourier hologram formed with these two sources is then $\begin{matrix}{{t(\eta)} = {{t_{o}\quad \ldots} + {\beta \cdot T \cdot \begin{bmatrix}{{\left( {{F(\eta)}_{1}} \right)^{2}\quad \ldots} +} \\{{\left( {{F(\eta)}_{2}} \right)^{2}\quad \ldots} +} \\{{\alpha \quad F{(\eta)_{1} \cdot {F(\eta)}_{2} \cdot {\exp \left( {{- } \cdot 2 \cdot \pi \cdot \eta \cdot b} \right)}}\quad \ldots} +} \\{F{(\eta)_{1} \cdot \alpha}\quad {{F(y)}_{2} \cdot {\exp \left( { \cdot 2 \cdot \pi \cdot \eta \cdot b} \right)}}}\end{bmatrix}}}} & (92)\end{matrix}$

where

F(η)1⇄f(η)1   (93)

and

F(η)2⇄f(η)2   (94)

If this hologram is illuminated with a plane wave, the complex amplitudein the output plane is proportional to the Fourier transform of t(η). Asbefore, the only term of interest is the third within the squarebrackets which, if we neglect a constant factor, is

ζ(F(η)₁ ·F(η)₂·exp(−i·2·π·η·b)=(f(y)₁ ·Δf(y)₂)·aδ(y+b)   (95)

If F(y)2 is identical to F(y)1, this term will result in a bright autocorrelation peak at y=−b. If, however, F(y)2 contains more than one suchcharacter f(y−c1)1, identical with f(y)1 but located at differentpositions and equal number of auto correlation peaks will be formed atlocations y=−b−c1, corresponding to the centers of these patterns.

In isochromatic electron-shading, this approach is very useful forproviding pattern and condition recognition both continual and real-timemonitoring (using non-invasive light) for targeted devices and operatingconditions in either temporal voltage waveforms, analogue and digitaloperations. This can be used to initiate various software and hardwarefunctions such as: initiating both “stop” and “load” codes, program andnode addresses, and initiating “link” operations for individual nodes(both star and cascade architecture's) in parallel processors. Thelimiting factors on system performance and operating bandwidth, are thephysical dimensions hence, optical transit time, the physical dimensionsof the voltage waveforms, and the compactly of the holographic storagemedium.

Data-Access Techniques

Local optical archival of information offers virtual immunity of binarycode for computers from degradation. For IR-based holography andoptoelectronic devices, holography provides storage opportunities tosimilarly record and preserve the spectral and spatial integrity oflight used for optical filters and waveguides.

The simplest approach of optical access employs a space variantapproach. Here, the location and position(s) of the input fielddetermines the composition of the output field from the hologram. Thebasic properties of such a system has been investigated for itsapplications in data processing.

To carry out a two-dimensional linear space-variant operation it isnecessary to have a system having impulse response is a function of fourindependent variables (two more than a normal optical system). Twomethods based on holographic techniques are described here.

The first is a simple method to perform the coordinate transformation

x=G ₁·(x,y)   (a)

y=G ₂·(x,y)   (b)

This transformation is effected with the optical shown in FIG. No.37(b), which uses a computer-generated hologram whose spatialfrequencies at any point (u,v) are $\begin{matrix}{{s_{u}:={G_{1} \cdot \frac{\left( {x,y} \right)}{\lambda \quad f}}}{and}} & (c) \\{s_{v}:={G_{2} \cdot \frac{\left( {x,y} \right)}{\lambda \quad f}}} & (d)\end{matrix}$

Light from a point in the input plane having coordinates (u,v) is thendiffracted at an angle such that an image of this point is formed in theback focal plane of the lens L2 at a point whose coordinates (x,y)satisfy (a) and (b).

More general operations can be realized, in principle, by a hologramarray. Each input pixel is backed by a hologram element which generatesthe desired response for the targeted device and voltage conditions.However, there are serious limitations on the number of devices andconditions which can be handled in this fashion due to the limitedresolution of hologram elements when targets are made very small.

Another method of obtaining a space-variant impulse response is to use athick holographic element such as shown in FIG. 38. This filter containsa number of superimposed holograms, each recorded with a plane referencewave incident at different angle. Each point on the input plane givesrise to a plane wave whose angel of incidence on the holographic filterdepends on the coordinates of this point and, hence, generates andimpulse response determined by the corresponding hologram. However, toavoid cross-talk, the input field must contain only a small number ofinput points, since all points on a cone satisfy the Bragg condition.

In FIG. 38 is an illustration showing the optical components forcoordinate transformation and beam pathways to produce a coordinatetransformation. In this approach each pixel of input produces a desiredresponse for that pixel, data or bus register.

Higher selectivity can be obtained by the use of coded reference beams.For this, a diffuser is inserted into the input plane, and each of theholograms in the filter is recorded with a reference derived from asmall area on this diffuser. Since the auto correlation functions of thereference beam are sharply peaked, each point in the input produces anoutput from the corresponding holographic filter. The diffuse backgroundarising from the cross-correlation functions of the elementary diffusersis minimized by using a thick recording medium.

Holographic Image Generation

Holograms or computer generated holograms surface mounted on eitherfrontside or backside can be made of IR absorbing materials to formtransmission holograms, in the case of reflection-probing of TO 224 IRreflective materials can be utilized on TO 224's backside so that aninterference pattern of the IC's internal circuitry and voltagetopography can be determined and resolved as to be in-phase orout-of-phase with the computer generated hologram. Options for mountingthe hologram are on the back substrate of the TO, mounted on a top layersubstrate over the circuitry, or integrated within the TO's device'selectronic circuitry by employing suitable semiconductor films whichappropriately absorb, reflect, or refract IR wavelengths as well assatisfying Bragg conditions. These holograms can be created directly onthe TO, or in proximity to, by conventional lithography techniques suchas using photomask reduction, ion-beam, e-beam and cyclotronic radiationsources, as well as implimention in free-carrier recording mediums.

Holograms created by computer calculations provide opportunity togenerate wavefronts of any prescribed amplitude and phase distribution;this has been demonstrated to be extremely useful for generatingthree-dimensional images, testing optical surfaces, opticalspatial-filtering as well as laser scanning. Production of theseholograms employs a discrete Fourier transform to generate the complexamplitude and phase distributions of an array of N_(n)×N_(n) elements.Each element of the image is broken into Fourier coefficients which arecomputed using the fast Fourier transform (FFT). In holography, thesecond step follows which produces a transparency (the hologram) whichreconstructs the object wave when suitably illuminated.

Many techniques have been developed for creating computer generatedholograms (CGHs) and holographic lens elements (HLEs). CGHs arerepresentations of the image's optical diffraction structure that hasbeen calculated from either a mathematical description (e.g., Fourieranalysis) of the wavefront or from samples. Several methods exist forcreating and calculating HLEs and CGHs.

In conventional approaches to performing CGH calculations, the amplitudeand relative phase of waveforms are plotted and optically reduced andtransferred to holographic film. Illumination of the developed filmcreates the three dimensional image of the object calculated forviewing. Similar rules apply for creating HLEs such as concave, convex,cylindrical and achromatic lenses, prisms, beam gratings, etc. See FIGS.19 & 20.

Replicating CGH plots by using a shorter (½ or ¼ of the IR wavelength)photogenerating wavelength so that free-carrier voltage waveforms orpatterns approximating optically by a 2-times enlargement of the image.This technique eliminates the need for photo reduction of amplitude andwave phases since the voltage wave patterns directly correlate to IR andUV optical interference waveforms. Also, the reflection brightness value(+70%) of the metal mirror surface is greater than the lighttransmission values of presently available holographic transmissionfilms (1% to 10%).

A Binary Detour-Phase Hologram has only two levels—either zero or one,the binary hologram is easily computed and projected onto the opticallyactive projection surface by an invasive optical beam. Projection doesnot require the use of a reference wave or bias other than non-invasivepolarized beam utilized to read the surface.

To produce the hologram, the surface area of the optically activeprojection surface is divided into array of N×N cells. Each cellcorresponds to the N×N coefficients of the discrete Fourier transform ofthe complex amplitude of the in the object plane. Each Fouriercoefficient is then represented by a single transparent area within thecorresponding cell, whose size is determined by the modulus of theFourier coefficient. This method derives its name from the fact that ashift of the transparent area in each cell results in light traveling alonger or short path to the reconstructed image. The effect here isachieved through the modulation of the TO material's polarization phaseor in the refractive of the image. An example showing both the cells andthe image of a typical binary detour-phase hologram of the letters ICOis shown in FIG. 39. The first-order images are those above and belowthe central spot; in addition, higher-order images are seen due tononlinear effects.

This method of encoding the phase works, by a rectangular opening (a×b)in an opaque sheet (the hologram) centered on the origin of thecoordinates, as shown on FIG. 20, and illuminated with a uniformcoherent polarized beam of light of unit amplitude. The complexamplitude U(x_(i), y_(i)) at a point (x_(i), y_(i)) in the diffractionpattern formed in the far field is given by the Fourier transform of thetransmitted amplitude and is${U \cdot \left( {x_{i},y_{i}} \right)}\quad:={{a \cdot b \cdot \sin}\quad {{c\left( \frac{a \cdot x_{i}}{\lambda \quad z} \right)} \cdot \sin}\quad {c\left( \frac{b \cdot y_{i}}{\lambda \quad z} \right)}}$

Illustration showing the configuration of the cells in a binarydetour-phase hologram shown in FIG. 39. The positions of the cells arecalculated in accordance to equations (84), (85) and (86);

(84)

where: $\begin{matrix}{{\sin \quad c\quad (x)}\quad:=\frac{\sin \cdot \pi \cdot x}{\pi \cdot x}} & (85)\end{matrix}$

We now assume that the center of the rectangular opening is shifted to apoint (Δxo, Δyo) and the sheet is illuminated by a plane wave incidentat an angle. If the complex amplitude of the incident wave at the sheetis exp[i(αΔxo+βΔyo)], the complex amplitude of the diffraction patternbecomes: $\begin{matrix}{{U \cdot \left( {x_{i},y_{i}} \right)}\quad:={{a \cdot b \cdot \sin}\quad {{c\left( \frac{a \cdot x_{i}}{\lambda \quad z} \right)} \cdot \sin}\quad {{c\left( \frac{b \cdot y_{i}}{\lambda \quad z} \right)} \cdot \left\lbrack {\exp \begin{bmatrix}{{{i \cdot \left( {\alpha + \frac{2 \cdot \pi \cdot x_{i}}{\lambda \quad z}} \right) \cdot \Delta}\quad x_{o}\quad \ldots} +} \\{{i \cdot \left( {\beta + \frac{2 \cdot \pi \cdot y_{i}}{\lambda \quad z}} \right) \cdot \Delta}\quad y_{o}}\end{bmatrix}} \right\rbrack}}} & (86)\end{matrix}$

continuing on . . . $\begin{matrix}{{U \cdot \left( {x_{i},y_{i}} \right)}\quad:={{a \cdot b \cdot \sin}\quad {{c\left( \frac{a \cdot x_{i}}{\lambda \quad z} \right)} \cdot \sin}\quad c\quad {{\left( \frac{b \cdot y_{i}}{\lambda \quad z} \right) \cdot \quad {\exp \left\lbrack {i \cdot \left( {{{\alpha \cdot \Delta}\quad x_{o}} + {{\beta \cdot \Delta}\quad y_{o}}} \right)} \right\rbrack} \cdot {\exp \left\lbrack {i \cdot \begin{bmatrix}{{{\frac{2 \cdot \pi}{\lambda \quad z} \cdot x_{i} \cdot \Delta}\quad x_{o}\quad \ldots} +} \\{{\frac{2 \cdot \pi}{\lambda \quad z} \cdot y_{i} \cdot \Delta}\quad y_{o}}\end{bmatrix}} \right\rbrack}}}}} & (87)\end{matrix}$

If axi<λz, byi<λz, equation (87) reduces to: $\begin{matrix}{{U \cdot \left( {x_{i}y_{i}} \right)}\quad:={{{\exp \left\lbrack {i \cdot \left( {{{\alpha \cdot \Delta}\quad x_{o}} + {{\beta \cdot \Delta}\quad y_{o}}} \right)} \right\rbrack} \cdot \exp}\left\lceil {i \cdot \left\lceil \begin{matrix}{{{\frac{2 \cdot \pi}{\lambda \quad z} \cdot x_{1} \cdot \Delta}\quad x_{o}\quad \ldots} +} \\{{\frac{2 \cdot \pi}{\lambda \quad z} \cdot y_{1} \cdot \Delta}\quad y_{o}}\end{matrix} \right\rceil} \right\rceil}} & (88)\end{matrix}$

If then, the computed complex amplitude of the object wave at a point(nΔx_(o), mΔy_(o)) in the hologram plane is

o(nΔx _(o) ,mΔy _(o))=|o(nΔx _(o) ,mΔy _(o))|·exp(iφ(nΔx _(o) ,mΔy_(o)))  (89)

its modulus and phase at this point can be encoded, as shown in equation( ), by making the area of the opening located in this cell equal to themodulus so that

a·b=|o(nΔx _(o) ,mΔy _(o))|  (90)

and displacing the center of the opening from the center by an amountgiven by the relation $\begin{matrix}{{\delta \quad x_{n\quad m}}\quad:={\left( \frac{\Delta \quad x_{o}}{2 \cdot \pi} \right) \cdot {\varphi \left( \frac{n\quad {\Delta x}_{o}}{m\quad \Delta \quad y_{o}} \right)}}} & (91)\end{matrix}$

To show the validity of this method of encoding, we consider the complexamplitude in the far field due to this opening, which is obtained bysumming the complex amplitudes due to all the N×N openings, is therefore$\begin{matrix}{{U\left( {x_{i},y_{i}} \right)}\quad:={\sum\limits_{n = 1}^{N}{\sum\limits_{m = 1}^{N}{{{{o\left( {{n\quad \Delta \quad x_{o}},{m\quad \Delta \quad y_{o}}} \right)}} \cdot {\exp \left( {{\alpha\delta}\quad n_{n\quad m}} \right)} \cdot {\exp \left( {\begin{pmatrix}{{\alpha \quad n\quad \Delta \quad x_{o}\quad \ldots} +} \\{\beta \quad m\quad \Delta \quad y_{o}}\end{pmatrix}} \right)}}{\exp \left\lbrack {\begin{pmatrix}{{2}\quad \pi} \\{\lambda \quad z}\end{pmatrix} \cdot \begin{pmatrix}{{{nx}_{i}\Delta \quad x_{o}\quad \ldots} +} \\{{{my}_{i} \cdot \Delta}\quad y_{o}}\end{pmatrix}} \right\rbrack}{\exp \left\lbrack {\begin{pmatrix}{{2}\quad \pi} \\{\lambda \quad z}\end{pmatrix}\delta \quad x_{n\quad m}} \right\rbrack}}}}} & (92)\end{matrix}$

If the dimensions of the cells and the angle of illumination are chosenso that

αΔx _(o)=2·π  (93)

βΔy _(o)=2·π  (94)

and

δx_(nm)=λz   (95)

equation (92) reduces to $\begin{matrix}{{U\left( {x_{i},y_{i}} \right)}\quad:={\sum\limits_{n = 1}^{N}{\sum\limits_{m = 1}^{N}{{{o\left( {{n\quad \Delta \quad x_{o}} + {m\quad \Delta \quad y_{o}}} \right)}} \cdot {\exp\left( {\quad {\varphi\left( \quad {{n\quad \Delta \quad x_{o}},{m\quad \Delta \quad y_{o}}} \right)}{\rbrack \cdot {\exp \left\lbrack {\begin{pmatrix}{{2}\quad \pi} \\{\lambda \quad z}\end{pmatrix} \cdot \begin{pmatrix}{{{{nx}_{i} \cdot \Delta}\quad x_{o}\quad \ldots} +} \\{{{my}_{i} \cdot \Delta}\quad y_{o}}\end{pmatrix}} \right\rbrack}}} \right.}}}}} & (96)\end{matrix}$

This is the discrete Fourier transform of the computed complex amplitudein the hologram plane, or in other words, the desired reconstructedimage.

Illustration of a typical cell in a binary detour-phase hologram showingthe configuration of a single cell in the binary detour-phase hologramshown in FIG. 22. The spatial relationships denoted by the variables areutilized in equations (87) through (96).

Binary detour-phase holograms have several attractive features. It ispossible to use a simple pen-and-ink plotter to prepare the binarymaster, and problems of linearity do not arise in the photographicreduction process. Their chief disadvantage is that they are verywasteful of plotter resolution, since the number of addressable plotterpoints in each cell must be large to minimize the noise due toquaintization of the modulus and the phase of the Fourier coefficients.When the number of quanitization levels is fairly large, this noise iseffectively spread over the whole image field, independent of the formof the signal. However, when the number of phase-quaintization levelsare small, the noise terms become shifted and self-convolved versions ofthe signal, which are much more annoying.

Generalized Binary Detour-Phase Holograms

In this method, as shown in FIG. 40b, rather than producing a singletransparent area with a variable size and position in the cell,corresponding to each Fourier coefficient, a combination of p×qtransparent and opaque subcells is used. This method permits finerquaintization of both amplitude and phase, resulting less noisy images.However, it is necessary for the computer to identify the proper binarypattern out of the 2^((p)×^(q)) possible patterns, that is the bestapproximation to the desired complex Fourier coefficient, beforeplotting it.

FIG. 40a shows a typical cell in a generalized binary detour-phasehologram and the arrangement of elements within a typical cell in ageneralized binary detour-phase hologram.

Phase Randomization

The Fourier transforms of the wavefronts corresponding to simpleelectronic circuits and their voltage levels have very large dynamicranges, because the coefficients of the dc and low-frequency terms havemuch larger moduli than those of the high-frequency terms. This resultsin nonlinearity because of the limited dynamic range of the recordingmedia.

To minimize this problem, it is convenient, where the phase of the finalreconstructed image is not important, to multiply the complex amplitudeat the original sampled object points by a random phase factor beforecalculating the Fourier transform. In transmission holography, this isoptically analogous to placing a diffuser in front of the objecttransparency and has the effect of making the magnitudes of the Fouriercoefficients much more uniform, as shown in FIG. 41a. However, thereconstructed image, FIG. 41b, is then modulated by a speckle pattern.

The Kinoform

In the case where the object is diffusely illuminated, the magnitudes ofthe Fourier coefficients are relatively unimportant, and the object canbe reconstructed using only the values of their phases. This led to theconcept of a completely different type of hologram called a kinoform.

This a computer generated hologram in which all the cells are completelytransparent so that the moduli of all the Fourier coefficients arearbitrarily set equal to unity, and only the phase of the transmittedlight is controlled in accordance with the phase of the computed Fouriercoefficients. Thus, the amplitude transmittance tnm of the cellcorresponding to a Fourier coefficient with modulus Onm and phase φnmwould be:

t_(nm)=exp(iφ_(nm))   (97)

However, to simplify recording, integral multiples of 2π radians aresubtracted from the computed phases, so that they vary only between 0and 2π over the entire kinoform.

Kinoforms have the advantage that they can diffract all the incidentlight into the final image. However, to achieve this, care is necessaryto ensure that the phase matching condition expressed by (97) issatisfied accurately. Any error in the recorded phase shift, as alteredby subsequent non-functionalities from the ideal test values of the TO,results in light diffracted into the zero order which can spoil theimage and thus signal the presence of test failure conditions beingpresent in the TO.

FIG. 19 shows an example of a Kinoform, a computer-generated amplitudephase hologram. In this approach, the magnitudes of the Fouriercoefficients are relatively unimportant since the computer calculatesthe image illustrated in FIG. 19 from the reconstructed values of thephases of the hologram shown in FIG. 19. The hologram's phases are firstplotted using a printer, then it is photo-reduced by conventionallithography techniques or directly etched by ion-beam, e-beam, orcyclonic radiation sources, or by free-carrier medium for illuminationof the image through a diffuser. This technique has been applied tothree-demensional image generation.

Computer Generated Interfergrams

Problems can arise with detour-phase holograms when encoding wavefrontswith large phase variations since, when the phase of the wavefront movesthrough a multiple of 2π rad, the two apertures near the crossover mayoverlap. This has led to an alternative approach to the production ofbinary holograms based on the fact that an image hologram of a wavefrontthat has no amplitude variations is essentially similar to aninterferogram, so that the exact locations of the transparent elementsin the binary hologram can be determined by solving a grating equationwhich correlates with the TO circuitry and operating voltage parametersof the test.

Different methods can be used to incorporate information on theamplitude variations in the object wavefront into the binary fringepatterns. In one, the two-dimensional nature of the Fourier transformhologram used is used to record the phase information along thex-direction, while the fringe heights in the y-direction are adjusted tocorrespond to the amplitude. In another, the phase and the amplitude arerecorded by the position and width of the fringes along the direction ofthe carrier frequency, while in the third, the phase and amplitude ofthe object wave are encoded by the superposition of two phase-onlyholograms.

This approach can be duplicated by plotter and photo reduction, orcontentionual photolightographic, techniques as well as implimention ina free-carrier recording medium. Here the computer calculates thehologram at two levels of its amplitude transmittance—either one or zero(top). The reconstructed image appears in the lower illustration FIG.40b.

Creating the voltage patterns with invasive light is accomplished byoptically retrieving indexed patterns (matrices) that correspond to x,y, and z, coordinates of the holograph image field to be generated aswell as Fourier transform functions. It is important to note in creatingoptical grating patterns metal mirror's photo-active surface usingvoltage waveform patterns, that the wavelength of electrons is less than100,000 times shorter than visible light so no photo-reduction isnecessary. The electron shadow that is created on the photovoltaic cellproportionally corresponds 1:1 to the interference lines of the realimage upon the electro-optic surface. In this application, theisochromatic element serves as a mapping filter of the voltagewaveforms.

One approach to display a holographic image for viewing is to generatemultiple voltage patterns so that they are transposed beside each otherin such a way that they bisect the active optical surface and interferewith each other in parallel to provide a parallax composite image.

In effect, the image's optical targets or input registers can also serveas index keys to retrieve multiple stored optical patterns that serve asprecalculated optical patterns (copyrightable). These elements serve astemplates to create the complex features (multiple Bragg cell conditionspresent) of the electro-static surface topographic electro-optic surfacethat goes into making three dimensional holographic images for displaywithin a defined field (e.g., 6 mm×6 mm [width]×3 μm [depth]).

The inventor then realized that the abilty to resolve supra resolutionin CCD detector devices disclosed in FIGS. 14a/14 b/ 14 c/etc. by usingholographic microscopy techniques, enabled the creation and developmentof new microwave frequencey optoelectronic descrete devices.

The inventor then realized that the ability to resolve supra resolutionin CCD detector and sensing devices previously disclosed in FIGS. 14a/14 b/ 14 c/etc. by using holographic microscopy techniques, enabled thecreation and development of new high frequency microwave multivalueddescrete optoelectronic devices. The large (roughly 26² μm) detectorpixels in the average CCD device contrast with the presentstate-of-the-art in semiconductor microelectronic line and device (e.g.,0.014 μm and the pending 25 nm) lithographic techniques. The inventorthen realized that the dimensions of high frequency microwave devicesare limited not by the available lithography but, rather by theresonance of the electromagnetic waveform or carrier signal itself whichpropagates through the individual electronic components in the microwavedevice.

In contrast to conventional lithography techniques and designconventions where more expitaxal devices are packed closer together ontoa single chip to form more complex semiconductor devices, the inventorrealized that the solution to developing higher operating frequencieswas not in creating more descrete exptitraxal devices but, insteadcreating individual devices which would have more functionality if onecould be able to resolve and alter the dynamic free carrier structurewithin a single, simple or complex expitaxal device. Instead of eachbinary expitaxal device modulating between two voltage levels, thedevice would be host to an array of voltage patterns. These descretedevices can be subsequently joined operationally together to formcomplex devices and systems which are disclosed later. By fashioningcomponents in this manner it would be possible to create devices whichin term would be equivalent to billions and/or trillions of conventionalexpitaxal devices are simulated within the device's free-carriers aseither digital words or matrices, analogue waveforms, or symboliccharacters. Timing of the free carrier patterns is controlled bymicrowave signal waveforms within the circuit and by discrete infrared(semiconductor materials)and ultraviolet (free-metal materials)holographic filters, beamsplitters, and holographic optical elementsdisclosed earlier. The free carrier patterns can be invasively writtenusing specific photovoltaic wavelengths of light to the device'smaterial, or by other sources of electromagnetic radiation such asx-rays, UV-light, magnetic fields, and other circuit devices inproximity or working in conjunction to the device. The functionalelectrical characteristics and physical dimensions of the free-carrierhost device determine its operating frequency, phase cycle, samplingperiod window, stability or lifetime of the free-carrier pattern(s), andsubsequent rewrite cycle.

The inventor then realized that the electrical characteristics at p-njunctions would serve as an effective free-carrier host device.

Forward-Bias Processes

Band-to-Band Tunneling

Photon-Assisted Tunneling

Injection

Tunneling to Deep Levels

Band Filling

Optical Refrigeration—optical pulse modulation and control systems fortemperature control, good until approximately 30° K.

Hetrojunctions

Zener Breakdown

Avalanche Breakdown

Photoelectric Emission

Effect of Surface Conditions

Photovoltaic Effects

The solar or fast photocell

The Schottkey Barrier

Photovoltaic Effects at the Schottkey Barrier

Bulk Photovoltaic Effects—Dember Effect

Photomagnetoelectric Effect—useful for supra-resolution of magneticfields in MRI devices.

Angular Dependence of Photovoltaic effects—useful for multivalue logicdevices with multiple inputs and outputs on a single substrate and onmulti-substrate systems.

“Photangular Effect”—useful for multivalue logic devices with multipleinputs and outputs on a single substrate and on multi-substrate systems.

Optically Induced barriers

Photovoltaic effect at a graded energy gap

Photopiezoelectric Effect

Macro-device host circuits—utilize microwave multvibrator and oscillatorelectronic circuits and macro devices having optical path and circuitphase/mode matching.

SAW devices—signal processing and conversion

Optical associative memories

Optical pattern & condition recognition

UV reads free metals, photowrites to IR

Four-wave mixing in free-carrier mediums which target character andvoltage specific conditions in infrared stimulated luminescence,carriers can be excited out of traps by optical excitation. Since inmost semiconductors the depth of the trap is less than 1.5 eV, theexcitation can be obtained by infrared illumination. After the carrieris excited out of the trap, it makes a radiative transition, emitting aphoton hv. E_(t). Hence after the semiconductor crystal has been“pumped” at low temperature, the luminescence which occurs when thetraps ate emptied by IR radiation. The depth of the traps are determinedby the spectrum of the incident optical radiation. The inventor thenrealized that if a material such as ZnS were used as an IR opticalreceiver for reflective or transmitted radiation from a free-carrierhost device or a free-carrier host array, visible, invasive orphotoenerating, radiation luminescence can be stimulated at lowtemperatures when the material is illuminated with 1.2 μm radiation. Thephotgenerating radiation can be subsequently directed back tofree-carrier host to invasively write a free-carrier pattern whichfunction as an optical register hold, or can be directed as shiftregister carry-over to another free-carrier host device. The invasivebeam from the ZnS surface can also serve as an object beam to aholographic associative memory and/or holographic pattern recognitiondevice which would subsequently write an invasive free-carrier patternwhich would subsequently as a logical or analogue function as itinteracts with other free-carrier fields resident with the host device.The inventor then realized that IR quenching of luminescence can beaccomplished in some materials during the afterglow of phosphorescenceor during excitation. Quenching of illumiscence can result from theapplication of an electric field, from heating, or from illuminationwith infrared radiation. Depending upon the characteristics of thematerial, the radiative transition may become filled, or a non-radiativepath may become available. In ZnS a broad band of radiation at about0.75 μm always quenches the flourence and phosphorescence, but at 1.2 μmwhich always quenches the luminescence at room temperature, it can alsostimulate the luminescence at 77° K. Applying an electric field inconjunction with IR radiation provides better timing control of the ZnSmaterial in integrating it within a free-carrier device system or anoptoelectronic array.

SUMMARY

The present invention relates to improved voltage test systems.

What is claimed is:
 1. A method of testing a device under testcomprising: (a) providing a beam of light from a light source having afirst wavelength; (b) imposing said beam of light on a test device overa spatial region within said test device substantially greater than saidfirst wavelength, wherein said test device has a first state ofrefraction; (c) imposing said beam of light on said test device over aspatial region within said test device substantially greater than saidfirst wavelength, wherein said test device has a second state ofrefraction; (d) obtaining data resulting from the interference of saidfirst beam and said second beam within said device under testrepresentative of the voltages within said region; (e) wherein saidfirst state of refraction is at a first voltage potential, and whereinsaid second state of refraction is at a second voltage potentialdifferent from said first voltage potential.
 2. The method of claim 1wherein said beam is provided from a laser.
 3. The method of claim 1wherein said coherent light is infrared.
 4. The method of claim 3wherein said test device is silicon.
 5. The method of claim 1 whereinsaid interference of said first beam and said second beam is within saidtest device.
 6. The method of claim 1 wherein said interference of saidfirst beam and said second beam is calculated.
 7. The method of claim 1wherein said first state of refraction is without a voltage beingapplied thereto.
 8. The method of claim 7 wherein said second state ofrefraction is with a voltage being applied thereto.